A T1 is basically a straight through CAT5, when thinking about it in simple terms. I don't know if you've ever experimented with sending 2 subnets over a CAT5, but it works. You really can send as many subnets over a cat5 as you have of ports on a switch on each side of the CAT5. You can have one PC on one subnet and it will talk to the PC on the same subnet on the otherside of the cable. Then on the two sides, put another PC on another subnet and put another PC on the otherside of the cable on that same subnet and the second two sets of PCs could talk. But subnet1 PCs and subnet2 PCs would have no way of communicating with eachother without a router that could route between the two subnets.
So, if you have it setup with your two buildings so they're on seperate subnets, I don't see why you couldn't have a switch on the headend and a switch on the far end. Then you'd have to have a router on the head-end that would route between the original T1 subnet and the DSL subnet but both subnets would be riding over the same T1 line now.
I guess I never did anything like this with a router. I have setup two subnets on one CAT 5. More I'm thinking about it, the routes are going to get weird with both routes being on the same port. But most routers, you can specify the source network, destination network and then the next hop far end IP. So, I'd try setting it up. You would have legit next hop IPs. They'd both be on the far end of the same T1, but the router doesn't need to know or care.