There’s an easy way to fix this. The Pidgin iOS app could be forked into a different licensing model. It also need not be open source, just demonstrate feature parity. Another way is to modularize the core messaging layer under a BSD license, and have the desktop app and iOS app both implement the front-end (and OS specific functionality) under distinct licenses.
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.
There are many ways to skin this cat.