I know for a fact the 6400 can output at 512 x 384, as I once connected one to a 12" RGB (aka LC Monitor). However, the CRT of the compact Macs has a resolution of 512 x 342.

I feel your best bet would be to find a small CRT to transplant. I seem to remember a thread from some time ago about tiny CRT monitors, so I'm going to follow Bunsen's example and recommend a search or two for that.