I know it doesn't help with the player you already have, but having an MP3 file pickup where it left off on power-down is one of the main features I test for when buying an MP3 player. If that doesn't work, the player goes back to the store. The Creative Muvo line (the little ones that run on a single AAA-cell) supports this, as does the iPod Nano (and I assume all iPods).
One thing about those non-MP3 audio books is that they seem to support user-selected bookmarks (at least on an iPod). I don't know if that is a feature of the audio file or the player. This doesn't do me any good, as I always refused to use anything but MP3 files, due to DRM issues. Looks like this is changing.