I just finished a build following the instructions on tinymail.org, but my N800 isn't here with me, so I won't be able to give it a try before this evening

I think the build should be fine. Modest didn't work in emulator as described on that page, but tinymail demoui did. If you have OS2008 installed and want to give it a try you can fetch the tarball here. Setup on the tablet is described on the bottom of the page linked above - basically you have to unpack and copy the message file to /usr/share/locale. Please report back, I'm curious!

DISCLAIMER: I give absolutely NO GUARANTEE about this software. Infact I haven't even tested it. Don't come crying to me if it deletes all your mail or blows up your hardware.