solar panel coverage over entire roof?

Yes different jurisdictions have fire code provisions that vary. check your local Authority Having Jurisdiction for particulars.

Know your codes for solar mounting (solarpowerworldonline.com) This article covers the various codes and standards that may apply.

Basically there is no technical reason for not covering the entire roof but there are codes that may prevent you from covering it plus there is the practical matter that panels come in standard sizes. Additionally its rare to see a roof that does not have something sticking out of it which can cause shading.
Another way to look at it is ground mount systems that do not require firefighter access cover 100% of the "roof"

Note at some point the total connected KW may exceed a threshold of connected solar to the local utility. Many states have "short form" permitting for small residential systems, go over the threshold and the cost and complexity for an interconnect can take years and can be very costly.

Besides many localities having restrictions on how much of a roof can be covered by panels as well as the restriction of panel/array placement with respect to which roof areas are off limits for emergency personnel access/safety, there are also servicing considerations.

How do you get at a panel in the middle of an array if it or its wiring/optimizer/micro needs service ? Won't need service ? Think again.

Another big concern: Cleaning. Won't need cleaning ? Think again.

Think long term.

I think cleaning is not needed

You can do it your way ! I still hose mine off 1 or 2 times in the summer from dust. (in the mornings when the panels are cool

It would typically involve loosening four bolts and possible laying carpet or other material on the panels that are used for access. Not as easy as one on the edge of the array but doable. There has to be a tradeoff between an efficient layout and serviceability.

WHAT ??? and you would put body weight on a row of good panels to get to a bad one ??? I'm sure that leaves an unmistakable stress pattern on the edge seals and is not good for the wafers you press your elbows and knees on.

If I or anyone else was worried about that we could also use plywood walk panels. That would be the "other material" I referred to in my post. Of course there is always the alternative to move all the panels that are in the path of the maintenance worker. That would avoid the unmistakable stress pattern on the edge seals and protect the wafers. In ten years with five different micro inverter or optimizer systems I have not had an issue that would drive me to want a less efficient layout. I have not noticed any trend in that direction by any installers on other pitched roof installations. On flat roof installs the efficiency gained by tilting the panels often results in separation between rows. Other readers are free to find their own solutions.

In my engineering opinion, using plywood as you suggest is a bad idea. Because it flexes, plywood wouldn't have much of a chance of avoiding the possible damage Mike speaks of. At best it's a crap shoot.

To maybe help see why, try a thought or a real experiment: Put 2 sawhorses about 65" apart and lay a 3/4" piece of plywood on them. Then, stand on it at the midpoint while someone measures the deflection caused by your weight. On a panel, the load will be distributed between the panel edges and over the glazing, mostly at the center of gravity of the applied load. At the point, that deflection can be considered a measure of the amount of force that will be transmitted to the glazing and then to the cells. Also, and to make things worse, walking or moving on a surface amounts to a variable applied load which can intensify the stresses and have some of the characteristics and effects of an impact load.
At best, you'd need a material with rigidity from strength/shape (section modulus) so that it won't deflect enough to cause the underside of the material to touch the glazing, not to mention what the loading (dead and variable) might do the the cells at the edge seals of the panel.

Then too, as a real world consideration, how will the plywood (or anything else for that matter) be restrained from sliding in a practical, workable way ?

Array layout is always about design tradeoffs, panel accessibility being one consideration. Given that micros and optimizers seem to violate the kiss principle more than string inverters, I'd suggest that non string inverter systems would seem to play a larger part in array layout considerations with respect to serviceability, but I doubt most potential PV system owners have a clue about such things and I'm pretty sure most peddlers don't either, much less consider it in array layout/design.

I consider your related experience anecdotal, especially when most mfgs. and peddlers tout or at least strongly imply 20 + years of trouble free service.

Wait a few years when all these devices fastened to the backs of hot during the day/cold at night panels that were so easily installed start to fail one by one. 20 years can be a long time.

I agree that anyone with the same concerns, should plan an alternate method for access or have a different design.

This is why I dislike the poor advice you often share here. You state something ignorant/reverse of the panel warranty ( weight loading ) and then when confronted, produce a quote like the one above.
Neophytes here to learn, see this stuff, and "wow, I read this on SPT so it must be good" Panels, batteries, just about every aspect of a PV system, you promulgate poor ( sometimes the poorest ) choices. Unless a beginner reads all the way through and sees the followups, they are going to follow footsteps into the quicksand. Oh well, then plan an alternate method.

Kind of like a loose cannon ?? What will be the outcome and fallout from the next Ampster loose cannon event ?

If you are planning to cover the entire roof with a solar panel then you have to think about other factors also. Only installing solar panels is not enough. After the installation process, there are many things that need to be considered. If you cover the entire roof with a solar panel array then you may face problem while cleaning the panel surface. As the entire array is not reachable you can not clean it in an appropriate way.

Also, using plywood is also not a good idea as it's durability is very less, you may need to change it in the future.