Hey again. You probably don't hack anymore (judging by the date of your last visit) but I've started working on a remake of my first hack earlier this year and I noticed all the useless mail items in the TrickHouse in Emerald (they don't do any good with emulators). If you clear all of those out, that gives you 9 great flags to use. It isn't much really, but it helps.
By the way, the flags from around 0x200 and up that are said to be useable don't always work correctly since they are probably used by something else already. If you want more good ones, you could also try setting the Voltorbs in New Mauville to reset their positions on re-entry to the map by giving them the same flags as the Cut trees so you can use their original ones. That also makes you able to battle them again if you re-visit that area.
Yeah, I tested those flags a few times each (about 40 of them or so; 20 coming up from 0xAAAA and 20 down from 0xFFFF) with a small plain script and they all work as intended. Just as a little precaution since I can be paranoid, I'd use 0xAAAB to 0xFFFE since I'm a bit suspicious of using values that have all the exact same numbers/letters. Ones with all same numbers might be hidden disposable flags or something but these ones here always worked right when I tested them.
I thought of this idea while I was trying to figure out what flags to use besides 0x1000+ because 0x1011 and 0x1111 turned out to be somehow in use already in a clean rom when I got to it and it made me wonder about the others. I don't know about extended flags, especially for FireRed as I pretty much never hack it.
^spoiler'ed to make this big post shorter.
EDIT: Actually, I just got some news yesterday from Diegoisawesome that it may in fact be somewhat risky to use these values as flags. Apparently the game's flag reader cannot tell if these are really flags, but uses them as such anyway. Now I'm not saying they will cause problems, but I'd advise you to use them with caution if you insist on using them.