Well, if you have any perl experience or can talk someone who does into writing you a little perl program this is quite easy. I tend to write custom scripts in perl to do my ID3 tagging because I have never found an ID3 tagger for unix that is automated enough. The two perl modules you want to look at are MP3::ID3v1Tag and MPEG::ID3v2Tag.

I am sort of in the middle of writing a little perl script that sets v1 and v2 tags based on existing tags if they are found (with v2 taking precedence if they both exist) and falls back to using directories if tags don't exist. It prompts the user to verify each tag, but usually you just have to keep hitting enter.

-Mike