I suspect folks don't understand what you mean. by "transom rubber". Is this the rub rail over the hull to deck join? The termination of this rub rail? or is it the plastic which covers the Hull to deck joint at the top of the transom?
If it is the latter, I would start by using a heat gun very carefully to heat the end you wish to remove,and then slide a thin piece of metal (if you can heat this when heating the plastic, that will help. Gently lift the plastic, then slide further. Starting with some debond will make the process easier. C Dory has a history of using Bostik adhesives in this time frame. Bostik make a debond for urethanes. There is also "Debond" which I know works for 5200 and similar 3M adhesive sealants.
Using debond can be a long process. I had to remove some teak cleats which the prior owner of the last CDory 22 had glued to the galley counter top. I used Debond, and a thin steel old guitar string to work under the teak and atatck what appeared to be 5200. When I have put the plastic trim back on the top of the transom, I used 5200 and it worked well. Bostik make a sealant similar to 5200.
