I'm sure you're probably aware of rec.martial-arts. Here's its Frequently Asked Questions list in HTML format.
A newsgroup called alt.arts.bujinkan was just announced, and promises to be useful for students of ninpo. If this group doesn't show up on your server, it may simply be because it's new.
The newsgroups rec.knives and rec.guns occasionally have self-defense and martial-arts related threads as well, but do not exist primarily for that purpose.