I fail to see how that has to do anything.

All it takes is making something auto-set the rotation-enabled flag to on. Some daemon/script should be able to do it, if you look at how Shortcutd does it. Could be pretty resource heavy though - but if you made it do nothing than respond to dbus signals, you could make it happen. I mean really, if you just combined the source code that allows shortcutd to enable rotation with the source code of rotate daemon, it could be done.

At any rate, I'm kinda assuming/hoping the full-scale portrait-ization of Hildon will happen soon, now that MohammadAG fixed the Qt bug that made it a problem and submitted the fix upstream. And then we won't have to worry about finding ways to enable auto-rotate on everything.

Actually, you can probably do it by using a Queen Beecon Widget which launches Conversations, then uses dbus to enable auto rotate on the just launched app. Or dbus scripts that listens to the launch of Conversations on dbus? One of those.