The thing was that I was forcing Gajim to do a transport-replace right after the jingle session initiates. That was good for testing, but in the real world scenario, transport-replace would only come after Gajim recieves the following stanza:
<iq email@example.com/bar' id='hu3vax16' firstname.lastname@example.org/foo' type='error'> <error type='cancel'> <item-not-found xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> </error> </iq>
I encountered an array of minor problems related to the JingleFT branch, that have prevented me from successfully test for the IBB fallback. Hopefully they will be sorted out by the end of this week.