Good news: now Foobar (1.5.4 final) also sees/manages multiple TPUB and TLAN fields in mp3 files (ID3v2.3). If we fill in a new (single) field in Foobar (e.g. BPM or RG), the existing multiple fields will not be "corrupted" either (delimiter will not changed to semicolon). The only residual inconvenience: if we fill in a multiple field in Foobar (typical case: copying fields during FLAC> MP3 conversion).
However, one interesting thing: when (after file conversion in Foobar) i separate the temporarily merged multiple fields (in the FLAC files), $replace(%composer%,_ DPL _,\\) is not enough. In the Extended Tags Panel Mp3tag now sees: composer1=first\\second. Another plus operation is required (after): split field by separator "\". (A re-save would be enough, but it can not be solved with an action.) So, something has changed in the way, Mp3tag works.
Later: Mp3tag v3.06f solves the \\ problem.