First, let me say that MC's tagging support is second to none. Thanks for all the hard work in this area! However, it would be even better if it had the flexibility to explicitly map library fields to file tags.
In Options->Library and Folders->Manage Library Fields there is an option to "Store in file tags (when possible)", unfortunately, there is no way to specify exactly which file tag to save the library field to.
For example, the ID3V2 TENC (Encoded by) frame is used by some online music services and I would like to use it universally to tag where my files have come from. However, there is no way in MC to edit this tag. (I can read it just fine in the Tag action window.)
Some other ID3V2 frames that I've seen used and have no way of editing in MC are PRIV, TFLT, TSO2, TSOA, TSOC. TSOP, TSOT, TSRC, UFID, USER .... Some of these are used by tools like dBPowerAmp and some just come that way from whatever encoder used by online music services.
As far as implementation, this could be an additional field in Manage Library Fields as simple as a text field that allows the user to specify the tag (e.g., TENC) or it could be as complicated as having drop down selection of tags from various standards. For instance, I could select ID3V2 and get a drop down of frames to use.
For default mappings in MC, this would cleanly expose the mappings already in use. Instead of writing a test file by filing in all the tags and seeing what is written, it would be explicit in the dialog.