I tried this on a N800 that I recently reflashed with diablo and I didn't need to downgrade avahi or use chinook repos.

What I did was install the rtcomm beta BUT, on the rtcomm website the link to the installer is (http://rtcomm.garage.maemo.org/deb/r..._0.5_armel.deb). I did a quick check and the latest version is 0.7 (http://rtcomm.garage.maemo.org/deb/r..._0.7_armel.deb) which has proper support for diablo.
So I installed 0.7 and I added a link local account in Account settings and ran Pidgin in Linux on my desktop and added an "Bonjour" account and I set my status to be online on the tablet and both N800 tablet with diablo with rtcomm and ubuntu computer with pidgin saw each other.