This time, I found out that since even FCE Ultra is slow in X but runs perfectly in framebuffer, ZSNES also runs in framebuffer. However, when I start it up, the ZSNES WIP gives a flickering screen. It flickers up and down very quickly, so you can't really see the text that well, but the general image is there.
It says Press Spacebar to proceed. It doesn't work. Nor does everything else. Switching from VT7 (framebuffer zsnes) to VTx back to VT7 doesn't give the familiar ZSNES GUI, though. instead (and the colour is reproducible) I get a brown-orange line, with 6 columns... err... and it's still flickering.
Kinda hard to visualize. Let's see if fbgrab does the trick:
ok wait.

It was tinted orange-brown instead, but hey I'm glad it worked, at least.
I run 2.6.11-gentoo-r6, svgalib 1.9.19-r3, with vesafb and savagefb compiled into the kernel.
EDIT: No error messages given.