I don't thing McCain would be "hamstrung" by Republicans in congress. Odds are they will still be the minority, and yes, most of them can't stand McCain, therefore he could work with a majority of Dems, and those RINOs who feel the need. It would be like having a true independent in the White House.
Then again, there is the danger he upsets both sides too much, which would obviate any influence he had over congress.