Uh, James Wheare’s xspfy.com (spiffy) will “Convert XSPF to JSON”, including JSONP output, and has the special benefit of a liberal parser which is able to “Shrug off invalid XML with aplomb and grace.”
And I’m pretty sure I knew about it when James put it out there about a year ago, which goes to show that my XSPF Gate was the kind of reinvention where you remember the idea of a thing but not the thing itself and then decide you thought of it yourself.