Your idea sounds like a possible fix if done well. Make sure there's a lot of surface contact area. Whatever epoxy you use will have a suggested joint gap for best results. Typical for this stuff (ill fitting parts) is that a few spots will be just right and a lot more won't be, so having a lot of overlap will help the numbers. I like the use of tape as a dam for seeping epoxy, done the same decades ago in the ski shop world fixing skies.
Depending on the fit of the "new" post over the "old" I might consider a building up of the old's top to better fill the new's ID. Fiberglass, carbon fiber ribbon come to mind. By holding the old post in the frame one could file down the added layers fairly easily and reduce the gaps. Relying on the epoxy only to fill gaps much larger than parts of a MM is a poor idea. I have heard of people mixing into the epoxy fibers, short cuts of glass or carbon, to better fill the gaps. I would only suggest a slow curing epoxy to give you time to handle/assemble the two posts. Can you give us an aprox amount of overlap between length the two posts?
A seat post failure is one of the lesser concerns I would have when riding. Although I will say riding a seatless bike is a lot harder than most would initially think
Seriously we're not talking about highly stressed and structural frame or wheel parts. Broken bars, pedal spindles or cranks are usually a far worse thing to have happen. Andy