I've been using Foobar2000 to handle the FLAC to MP3 conversion, but I'm finding some of the tags I want aren't supported, e.g. RELEASETIME to ID3's TDRL, media TMED, trackid UFID.
I've also noticed that I have to change quite a few of MP3Tag's default mappings in order to get the ID3 tags converted properly (e.g. CONTENT GROUP rather than CONTENTGROUP, COMPILATION, TORY, all the SORT ones. . .)
So I'm looking for a different tool for this specific job - I realize I could script something myself, but would prefer a canned tool - CLI or GUI's fine, main features are large batches at a time, and as many tags as possible, ideally using MP3Tag's fieldnames, cover art, maybe even lyrics would be nice but not that important at this point.
If it turns out that one of the other "player / management" tools does this particularly well that'd be great, but am wary of letting other apps (than Picard and MP3Tag) mess with my personal tagging standards.
JJ pointed out Robin Bowes' flac2mp3.py, but I'd prefer something (more) actively maintained.