I should have given hardware specifics, sorry!
I have a Samsung UJ59 32". It apparently doesn't let you change the screen size.
My test machine has an nVidia GTX 710 in it, which doesn't seem to let you do anything with Aspect Ratio.
Interestingly, my work laptop with integrated Intel graphics does let me do scaling (maybe I should use it) where I can basically squeeze the vertical.
I have my full MAME machine on the way, which has a Radeon GX 580 in it. I'm hoping I'll get lucky with the AMD drivers.
I haven't tinkered with MAME, I didn't know if there's a way to adjust the size of its screen, or if Launchbox/other handled that. Worst case, I can probably get a custom bezel printed.
But, if anyone has a better idea, I'd appreciate it.