The MUC component available from http://download.gna.org/mu-conference/ apparently supports upgrading p2p chats to MUCs. http://trac.gajim.org/ticket/2095 is this feature in Gajim, which might be a useful reference.
This will need some spec work first, of course. RequestChannel 2.0 might be a blocker for this (so we can associate the two channels).
We can do this! <http://telepathy.freedesktop.org/spec/Channel_Interface_Conference.html>
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