All kanji work (at least i haven't found one that doesn't), the number is just the unicode code point. At least as far as being able to add stories anyway, i don't use the SRS on this site.
As long as the Kanji has a JIS code then it should be able to be added. I used IME Pad to test this. It isn't the most obscure Kanji though, but very obscure.
I think that's what I'll do (only for new kanji though, I pretend to keep reviewing the old ones here).
It still puzzles me the fact that you can add stories for non-RTK kanji, but you can't review them. I don't see the logic there. Unless you're dupposed to be able to review them and I'm doing something wrong or there is a problem with my account.
I checked the member list and people with the most kanji have 3030, which I guess are those from RTK 1+3 plus 23 kanji from the new supplement. So, it seems it's the norm and not a bug. Probably Fabrice intends to add non-RTK kanji support as a premium feature or something.