I don't recall if it was ever tested whether or not Mystic Arts from one weapon could apply to another weapon. Sword of Salvation, for instance says it extends the Timely Guard window, but doesn't mention if it requires a sword or not. It does say it requires a sword for the reduction of Ki consumption when dodging, but not specifically for the Extend Timely Guard aspect of the skill.
The main skills I'm concerned with are Sword of Salvation, Firm Resolve, Inner Light and Subtle Blade. I use Apocalypse and Shooting Star with Tonfa/Kusa, and I'm not concerned with the Spear MA's since they both clearly seem weapon-specific. But I need to know if, in particular, the ones I mentioned work with weapons outside of their tree to determine if they're worth the skill point investment or not
And you will all live to grow and thrive and lead happy lives...
...until old age take you back to the weeds.