Can I use pidgin-facebookchat/msn-pecan/(insert third-party prpl here) with Haze?

Kind of. Currently, you'll need to patch haze.manager, and add a .profile file for the protocol. (This is vaguely outlined on this bug report.)

Non-standard protocols will not be added to haze.manager. The .manager files are intended just to be a cache of what ListProtocols() would return if the CM was running. Since Haze uses libpurple, which loads plugins at runtime, this cache is basically broken. Sadly, current Telepathy clients depend on the presence of a .manager file for a CM, so Haze ships the best it can: the set of protocols shipped as standard with libpurple. This means that you can be reasonably sure that if haze.manager says a protocol's available, it'll be available when the CM's running. Adding non-standard protocols would break this.

Correct clients should react to a missing .manager file by firing up the CM and interrogating it. TpConnectionManager in Telepathy GLib now does this correctly. Sadly, Mission Control 4 (as used by Empathy) does this wrong. When Empathy moves to Mission Control 5 (which should be in time for Gnome 2.28) then this will all work and haze.manager can be deleted.

Sorry for any inconvenience caused.