The plan would be to affiliate in the reserves and finish up 20 in either SELRES (understand possibly not an option due to seniority) or IRR.
The OP isn't going to be competitive for a hardware unit Reserve job because he's too senior....That doesn't mean that one can't be a successful SELRES, it just typically wouldn't be flying.
Transferring to the SELRES after a DH tour is not common but certainly not unprecedented and definitely not an issue. We had an S-3 pilot who transferred to SELRES after his DH tour to join Delta and he just retired, so it certainly is possible. There used to be a rule/reg that you had to do 6 years in the reserves in order to retire but I am pretty sure that was done away with so even if you join at 15 or 16 years it shouldn't be a problem. The only catch is what Gator already mentions, flying for a hardware unit is not likely.