I can tell the current team is happy with XP, but they aren't whole-hearted devotees.

This is probably a good thing. After all, you wouldn't want all your projects looking like nails. See When should Extreme Programming be Used?

is XP self-sustaining or does it need a strong advocate inside the team to nurture and encourage it?

"Strong advocates" do more harm than good. Ensuring that the people who make the decisions know what extreme programming is and what its strengths and weaknesses are would be more beneficial.

chromatic wrote a good intro piece on the subject a little while back as well.