Oh yeah, that specific track is already in my playlist.
At one point he fell off the stage and hit his throat and lost quite a bit of vocal range. I think the WPLJ cover was before that. (I want to say Burnt Weenie Sandwich?)
EDIT: Wait, are you saying you don’t like Zappa’s cover of WPLJ? I love it, and I’m definitely not listening to the original you linked to if it’s going to make me not like it anymore, heh.
(And clicking your link to the cover I see it is indeed from Burnt Weenie Sandwich.)
Yes, very difficult to play while singing, but it should be pointed out that the OP is about singing and playing the same thing, which just about any player can do, IMO.
In the late 1970s when Benson was all over the charts, Guitar Player magazine had an interview with an older jazz guitarist (Jimmy Raney? Tal Farlow?) who said the technique was commonly used when he was young as an ear-building technique, to make solos more vocal-like.