I have been using something similar for a while. The LargeFieldsConfig.txt has the following changes:
basicMetaMax=8192 (to handle some longer lyrics: if this size is exceeded the field is not truncated but empty)
fieldBasic=lyrics
fieldBasic=unsynced lyrics
make sure to remove corresponding entries from the fieldSPam list
The field content formula is as follows:
$ifgreater($len(%lyrics%%unsynced lyrics%),20, $char(10003),$ifequal($strstr($lower($trim(%lyrics%%unsynced lyrics%)),instrumental),1,$char(12288)$char(9835),))
This indicates presence of either %lyrics% or %unsynced lyrics% by displaying a tick when the combined length is > 20 characters.
Additionally it will display a note character when the lyrics content is 'Instrumental'. I use this in the SimPlaylist which for some reason in my installation does not display the note character on its own, thus the hack to preceed it by a "fake space".