Franchising him does seem to me the only sensible option.
I don't think either side (Pats or Asante) would want it to come down to Samuel playing out the year under the tag, but it would give the Pats a little more time to negotiate with Samuel to get him resigned, as well as the opportunity to let him shop himself around for a trade. If neither could be worked out, the Pats could always waive the tag, as the Eagles did w/ Simon a few years ago... though I doubt it would come to that.
Samuel has an excellent season this year and has turned himself into a big-name player. As one of only two solid starting CBs on the free agent market, his trade value could be pretty high. I think there are numerous teams that would readily offer a 2nd round pick as well as a market-value contract to circumvent free agency, and guarantee they get their man.
Heck, if Branch could get us a 1st, there's a shot Asante could, too. Now that would be something...