I assume you are speaking about the MP3 format and the ID3v2 tags.
COMM tags were used for MC's own custom, aka user, tags until recently, but they were changed to TXXX tags because of better support in external programs. In addition some other programs destroy or combine multiple COMM tags that exist in a single file.
MC still has built-in support for some commonly used COMM tag frames in addition to the plain headerless Comment tag, but it can't read any arbitrarily named COMM frame. The developers must specifically add support for each COMM frame that has a different header.
Could you tag a small MP3 file with your tags (any MP3 sample/sound effect/demo clip would be fine - the audio content and quality would not matter), upload it somewhere and post a link here?
I would like to try if some other program could copy your custom tags to other, compatible tag frames.