I realized that in my fever for the other projects, that I had never actually seen The Last Jedi, so I remedied that.
At timecode 2:22:22 (what a cool coincidence...) in the movie we see it split down the middle with the crystal exposed.
This got me thinking. In the capture, the crystal appears to sit just about even with the end of where the retaining ring goes. If I cheat a little by sliding the retaining ring up, I can then fit 4 18650's and my oversized speaker, fully behind that in the lower handle. That gives me 14.8v-16.8v for driving the blade directly and enough Ah to run it for a good long time.
A little PWM will manage regular (full) brightness, with the ability to surge to 130% brightness for simulating strikes. Sounds like a plan.
This is also the best spot for them weight wise as it will balance out the blade much better than having them further up top. I think I might extend the lower tube so the joint is back in the middle, but measurement wise, those were the 'correct' dimensions. I just don't want to sacrifice the crystal chamber, or battery life, so this seems like the next best compromise.