How to make "poor man's roaming" by proxying cell calls to SIP or Skype?
There is an opensource application Sipdroid, which theoretically could be modified to do proxying. But, judging from the poor general Android SIP experience, I'd think it would be very unreliable solution.

Easier way to workaround greedy cell providers seems to set up an IP PBX on the home computer, or get a free account at freepbxes.org and get a cheap SIP-connected phone number to eventually route your calls thru while using your "Travel" device's SIP connection to terminate calls.