I too think this is a challenge. I suspect the only way to do this is have a 'curved rod' attached only at the walls. Or, a curved rod attached to the ceiling in the center-point with the curtain strung on both sides of the 'pivot point'. (This implies that the curtain opens from the 2 ends only.)
I think if you hit the Bed & Bath Stores as already suggested, you'll see some options. Heck, go to a Spa and Tub Retailer and tell them you want to buy a corner-installed whirlpool tub & shower. Then, look to ask about the rod system that comes w/it.
And at worst, if the guy above wants to draw up some plans for ya, you can't get a better deal! My best to ya and hope this helps.