If 60° was OK, you could make a cutter out of an old tap. Just leave one tooth "active". As Tom said, it would be slow, but cheap. A fine pitch thread would be more likely to have enough relief on a tooth to not drag because of the lead of the tap. Of you could just grind it to suit.
If you wanted to pitch the mill head over, you could do it with a normal end mill and elevate the table (on a knee mill) for each pass, and rotate it to do the other set. Time consuming, again.