[Geoff Bacon]

The only way I can think of is to use the "sources" metadata field and use  settings/library settings/song title formatting to display this field.

When the client has specified specific songs, edit the song metadata and set the sources to Requested or what ever you want.

After the gig, use a filter to identify/select all songs with this value and perform an single edit to remove this value from the selected songs (ready for the next gig) .  I suppose you could just select all songs (long press one song and use floating dialog to select all songs) then do the edit to remove the chosen word 

Someone else might have a better idea

Geoff