libpurple (the actual messaging library) is already modularised, many different people mantain[ed] different modules, it's been used my many third party messaging clients including Adium and multi-IM browser clients from those days like Meebo or imo.
It's not a commercial project with a CLA, it doesn't have a single owner, many of the people who contributed major parts have moved on or would object to a license change from copyleft to permissive, so it's not a matter of "just change" the license. And libpurple is the real meat of the project.
Meebo was the shit back then. (In browser GAIM, even fb chat, irc, etc).
I followed a few of the developers for a bit as I really wanted to work there. I believed they got aquired into the g+ team (gchat? maybe?).
There are many ways to skin this cat.