Sarah,
I know this probably doesn't help you much, but I just wanted to say, I've had my ileostomy for only three years now and while I did have some soreness and tenderness as you describe in the months following the surgery, I no longer feel a bit of pain when coughing, sneezing, laughing hard, etc. I just wanted to comment so that you know it may not necessarily be normal, i.e., there may be something that wasn't done exactly right. I know that I had an excellent G.I. surgeon at Pittsburgh UPMC hospital, would be happy to give the name via PM. Best I can do I guess, sorry can't help more.
I guess it might also have something to do with the location of the stoma, perhaps. Just thinking about it. My stoma is actually pretty high compared to what seems to be the "norm", mine is just below and an inch or two to the right of my belly button - or where my belly button used to be : Just a guess, certainly no doctor.
Rgds,
Smith