Some uneducated opinions:
1. So long as there is a degree of consistency, it should not matter what you choose. The only question should be if you intend to adapt your system to the OGL, or if you intended to adapt the OGL to your system, if you get my meaning.
2. What sounds better is probably best. That doesn’t mean that’s always going to be the case, but having to convolute your work on account of someone else’s generally renders a lackluster result. I personally think the original “martial feat” sounds good. “Martial skill” is also a nice option, but if semantics have been introduced which make these terms difficult, then perhaps you need to take control of the language being used here – refer back to point #1.
3. If you do opt to look for “free” or “available” terminologies, it might be best to determine which ones you CAN’T use. If you care to compile a list, that would help to determine what your options are by quite a good deal.
4. Ignoring what I don’t know about the OGL, here are some potential ideas:
“Martial Abilities,” or some other types of abilities, like “Combat Abilities,” etc.
If use of conventional words results in a wordblob, introduce some fresh new terms (I’m trying to be hip and failing at it). Maybe something like “Ars Martia” (some Latin stuff which I guessed at and apparently is mostly right) would do as a replacement. Also ties in with “Codex Martialis,” so perhaps that could be a winner. Also, something like that becomes your own “proprietary” term, so no matter when the OGL changes or a new one is introduced, you kind of lock in your own mechanics to the system regardless.
…That’s all I’ve got.