Perhaps something that piggybacked on an existing IM service's connection.

Perhaps you could have something that could do the following:

a) packetize your existing file into nice chunks
b) ascii armour it
c) spit the chunks over the existing IM connection as ascii data
d) reassemble the chunks on the other side.

You know, the xmodem idea isn't that far fetched. The only difference is that instead of dumping the data block over the phone line, you would be dumping it over a text IM session, probably proxied via http...

Still, it sounds like reinventing the wheel. If only the IM clients actually worked for sending their data files across the existing sessions without trying to open a different port...