As always, you should speak to lawyer about legal issues.
I would however strongly discourage you from a fork.
If a patch is rejected, it probably means you did it wrong,
designed it wrong or didn't explain it properly/fully.
It is actually a "well kept secret" in open source that the best
way to get a feature into an open source project is to post a patch
that tries to do something useful (even if it doesn't quite work properly).
If it really is a good feature there is a good chance the developer
with "throw away" your patch and code it properly :-)
The correct place to post patches is here:
not in the forums.