I feel you on this one... I've been trying to find out the same thing, myself, because I've heard a lot of conflicting reports on this too. I have been taking hormonal birth control for fifteen years... and I've heard murmurings that folks with port wine stains/Klippel-Trenaunay shouldn't take them because of increased risk for clots, etc. - but maybe they take it on a case-by-case basis, depending on the severity of the lesion. I have my annual soon; I'm going to take up the issue with my gyn and see what she says. I'll probably post about it here.
I wonder if there are herbal/holistic treatments you could try to help you with your symptoms? Maybe in conjunction with your gyn, doctors and perhaps an herbalist, you can come up with something that eases your pain a little? I'm not sure if you experience pain because of swelling, but I know that when I get swelling in my birthmarked foot, I sip something called PMS Tea (in the "hippie-food" section at the grocery store) and it does help the swelling a little, and definitely eases my cramps.
Keep me updated re: what you find out about hormones, and I'll try to do the same.