I also store some needed information, like a date of a life performance, in a already existing fields completely not related to it
And I had similar problem with M4A format: if I would write something like "1/4" in the DISCNUMBER, everything after the "/" wold get wiped out. And I could not use other field for that information, because it was one of those few that was displayed by Winamp in its File Components Area. And so I had to change to other lossless file format, so that all of the fields would behave exactly the same- save and show everything I put in them [and be shown under certain conditions in Winamp]. I managed to come up with a lossless combo of FLAC, WV and TTA [but there is only this one small problem with TTA: /t/19369/1]
It is a long shoot but maybe you need such revolution in your files, if the current reality does not suit you?