Nice find! xsel seems to be exactly what we need.
I just tried the script from ubuntuforums.org and at least for text it works. I also tried to copy images with it but unfortunately that didn't work. Instead I only got a single dot (the script's default empty content) when trying to insert an image. I'm not sure if the problem was on the copying or on the pasting side.
You'll find an optified version of Debian Lenny's xsel package (I just moved the files around, so there's no change in the source code) attached to this post. Install it under Maemo and follow the instructions at ubuntuforums.org!