I have no experience with the field derotator, but I chose not to buy it, even though it sounds mighty convenient--your concerns about back focus, etc., were among my reasons. Also,I sometimes do photography through a good short-tube refractor to get a wide field, and use the LX200 to guide it. At times I may choose to offset the refractor and the LX200 by a degree or two, in order to find a better guide star. I didn't know whether the field derotator would work well for that (maybe so, but I didn't want to risk it). I bought a wedge made by Ken Milburn of Bonney Lakes Astro Works( http://home1.gte.net/kmilburn/page2.html )--expensive and a long wait, but worth it.
The one thing Meade could do to make life better would be to put a polar-alignment scope on one of the fork arms. As it is, there are dozens of ways to get close quickly. For example, buy an extra Telerad (not the same Telerad you use on the scope itself if you have one. Also, not a Rigel Quickfinder, which is fine for many purposes but which doesn't reproduce precise alignment each time you put it back on). Put the Telerad on the fork arm for a few minutes early in the night. Take a few minutes the first night making sure the Telerad is precisely aligned with the fork arm (by being sure Polaris stays the same distance from the center when you rotate the polar axis, for example). Do careful drift alignment the first night. Then note where Polaris is in the Telerad when you center some other star (say, Kochab) in your eyepiece. Reproduce that every night, and you'll be mighty close, though not perfect--drift alignment will go quickly on subsequent nights. There are other methods you can use for precise, quick reproduction of drift-method alignment if you can set up over exactly the same spot every night (such as putting a cheap laser on your tilt plate so that the dot falls on some known spot far from the scope, as far north or south of the scope as possible). Using this method I've gotten good polar alignment in just a few minutes if I can set up over the same precise spot.
I'm a fairly new owner of an LX200 GPS 10-inch scope and am still learning. However, my initial impression is that the periodic error correction is indeed permanent in polar mode IF you are perfectly polar aligned.
Another big reason for using the wedge instead of the field derotator is one you mentioned. When you have a lot of accessories in the light path so that your camera sticks waaaaaay back, the camera can hit the base. In polar mode, so what? The only part of the sky that's blocked is the part within a few degrees of Polaris. How often will you photograph there? But in altazimuth mode, the part of the sky that's blocked is the part straight overhead, which you will want to photograph far more often.
--Daniel Johnson ("Dan")