I've used mayonnaise or vegetable oil on tree sap with great success - on dog fur, on leather.
I've never had to use it on clothing, so you may want to try a test area first, if possible. But it works wonders on other washable surfaces. You could probably use it safely and then use Shout or similar to get out the oil spot.
I had sap on a sweatshirt once, and just washing it without treatment got it out (hail the power of laundry detergent). But to be fair, it wasn't ground in at all, it had simply dripped on the surface and sat for a few days.