After writing it in a couple of afternoons and letting it wait for attention (alas, in vain) for months, I decided to release purplebridge so that others may play with it. Quoting from the wiki:
purplebridge is a GUI-less client for IM networks based on libpurple that disguises as a local XMPP server. Once ready, an XMPP client will be able to launch it, connect to it, and through it reach the foreign networks supported by libpurple.
The wiki has more information about status and rationale.