I think it's possibly a valid idea, but I'm sure there will be drawbacks (possible tag spam, etc.). I can't say how many times I've answered some type of question using some random combination of technologies which generally has the same solution but because the OP's ignorance of some subject, he/she has no idea what the real problem is.
I'm not sure that the tag soup should be the same as the questioners available tags, though.
Edit: Actually, I just realized the OPs request is different from mine. The OP wants to tag accepted answers. I do feel that's a bad approach (accepted answers aren't always the right answer). I wanted the ability to add tags to any answer. Sorry for the confusion.