The most interesting project of that sort which I've come across is Cody Brocious' Dyna6502Asm
which is a dynamically recompiled fork of 6502asm.
Note that with git, taking a fork is a natural way to do some development even if you expect to contribute it back. And on github, you can publish by maintaining a branch with the special name 'gh-pages' - as Cody has in this case
(It's not too hard to find a mail address for the author of 6502asm)
is worth a look too - there's an outstanding ticket for improving the 65c02 model, or you could venture into a GUI