I'm not sure I'd let that detail keep me from trying a Neiman, especially if the geometry is much better than the D9800 (which I'm assuming it would be). I'd probably set up the Neiman so 240 vertical lines filled up the screen, and then use MAME's slider controls to squeeze down larger resolutions so they also fit. You lose your pixel perfect ratio, but it'd probably still look pretty good.
It looks like ass. No reason to run native if yer gonna do that sort of thing.
Steph - if you want the convenience and all, go for a D9800. I had one for a little while, and it was pretty good. Not as good as my Billabs, but good enough.
In discussion, the dude at Billabs told me the 'switching' wasn't really intended for real-time, let alone frequent, use. I've had mine for almost three years, and have always run Windows at max res - XGA - done oodles and oodles of testing at all sorts of resolutions and stuff, have messed with the convergence, have given it signals it can't sync to (in which case, it has just shut down) - and to this day, it's in top form, both in spec and brightness. In fact, for the last perhaps six months, I've been using this as my only monitor. It's on all day, and it's still nice an perky!
Not to say the D9800 will come close to this performance, but if it's even half as good, it should be just fine for home arcade use for some years.