Derm is competitive, so yes, you should def have a backup plan and every interview will ask you what it is. A lot of ppl try to do a year of derm research if they don't match, but I don't think there is a right or wrong answer to that question as long as you do something worthwhile and ideally related to derm. I have never heard of any handshake deals or assurances for admittance.
As far as TRI's are concerned, I don't know of a list of the TRI programs other than what you can find on osteopathic opportunities website. Just try to go to a TRI with a derm program associated with it. They are basically all the same, I'd recommend looking into how many out electives they offer for derm auditions