Specialized doesn't sell their bikes online, so you'd have to buy it in a store anyway. That gives you plenty of time to look at the bike to see whether or not it has room for a cage.
Ah, what the hey, I love posting this picture.
This is the XS. I don't know the geometry off the top of my head, but I don't think any of my bikes are bigger than 15". Tons of room. I don't see why there would be *that* much less on a Stumpy.