The Filco Majestouch works great with Plover, even via USB:
http://www.amazon.com/Filco-Majestouch-2-Keyboard-FKBN104M-EB2/dp/B004VKUSG6
You definitely need more than 6-key rollover (16-key rollover is about
the minimum, I'd say, to be able to use the majority of the Plover
dictionary), but fortunately the Filco Majestouch has at least that.
Apparently the Noppoo Choc Mini works too, though I haven't tried it personally:
http://www.amazon.com/Noppoo-Mechanical-Gaming-Keyboard-Cherry-Switches/dp/B00A45WJZA
Nice thing about it is that you've got parallel (as opposed to
staggered/offset) columns of keys way at the top there (thanks to an
anonymous informant for pointing this feature out to me), so if you're
not a number bar user, you might be able to situate your stenoing way
up there. You'd have to play around with Plover's keymapping to make
it work, but it should be doable, if parallel columns are that
important to you. I'm a diehard number bar user, so it wouldn't work
for me, but hey. (':