Converting tags

It seems that the "standard" tags for FLAC format are some what limited with respect to
mp3 -- will mp3tag support non-standard tags that will map/relate to mp3 tag?

You can accomplish what you want by using Mp3tag's custom Tag Mapping feature. If you configure mappings like those shown in the following post then you can use standard FLAC field names to tag Mp3 files and they'll be mapped to the correct ID3v2 frames:

Album Artist, Band and FLAC, again ...

For example, with these mappings if you use ALBUMARTIST then it maps to Mp3tag's BAND, which Mp3tag then maps to TPE2. Use COMPILATION and it maps to Mp3tag's ITUNESCOMPILATION and that is mapped to TCMP.