In Loupe with Filmstrip showing: Select images within filmstrip, apply keyword in whatever way you like noting that keyword will attach only to Most Selected (1st selected) file, then Sync using "Check None" scroll to bottom and check Keywords (enter new KW here if you like).
Probably not what you were hoping for, but it does work. I use it when a round trip from Loupe to Grid to Loupe would take too long.
John-