I was only going to brown the butter and caramelize the sugar and then make a standard buttermilk pie apart from those tweaks. But while waiting for the caramelized sugar to dissolve in 2/3 of the buttermilk, I decided to substitute tangelo juice (and some of the zest) for the remaining buttermilk. (The buttermilk and tangelo juice almost seem to have separated into two separate layers.)
The result hasn't got much buttermilk flavor - no surprise, since one reason I leave out the lemon juice some people add to buttermilk pie is to keep it from covering up the much more subtle flavor of buttermilk. But it is sweet, tangy, tangelo-y, caramelly, certainly a success.