Maybe you want something like this?
LYRICS <== $if($eql(%_tag_read%,'FLAC'),%_filename%' (Lyrics).txt',%LYRICS%) UNSYNCEDLYRICS <== $if($eql(%_tag_read%,'ID3v2.3'),%_filename%' (Lyrics).txt',%UNSYNCEDLYRICS%)DD.20141110.1944.CET
Well, I think, the code proposal from above does not help, because it is crap !?!?
DD.20141111.1836.CET
I checked it again.
Mp3tag shows the tag-field UNSYNCEDLYRICS, ...
when it writes the tag-field USLT to ID3v2.x tag.
Mp3tag shows the tag-field UNSYNCEDLYRICS, ...
when it writes the tag-field UNSYNCEDLYRICS to FLAC/Vorbis Comment tag.
Mp3tag shows the tag-field UNSYNCEDLYRICS, ...
when it writes the tag-field LYRICS to FLAC/Vorbis Comment tag, ...
because of the internal mapping of field names, ...
see dialog "Mp3tag/Extras/Options/Tags/Mapping".
The FLAC tag get the internal Vorbis Comment tag-field LYRICS, ...
regardless of working in the user interface with the tag-field UNSNCEDLYRICS or LYRICS.
Therefore it is not necessary, to create different commands or formatstrings for storing unsynced lyrics for the different tag-types ID3v2.x and FLAC.
While working with the user interface of Mp3tag, both cases are treated in the same way, ...
while under the hood Mp3tag directs the lyrics data to their designated technical tag-fields.
DD.20150308.1915.CET