The joke as I see it is twofold - seems to me it's also referencing that Brent used to play for Silver (the band): http://allmusic.com/album/silver-r49370
"We (Brent) used to play for Silver (in '76, also on Arista), now (since Terrapin, '77) we (all, the Dead) play for Clive."
Dunno if it was a done deal by then (12/30/78), but... Keith & Donna were outta there like six weeks later. Brent came to the Dead via Bob, he played in Bob's band in '78.
I think this was a little advance clue Bob was dropping... prankster that he is. (Remember the 'Uncle Bobo' monikier he mercilessly foisted upon Bill Graham? It fits with Bob's sense of wordplay humor.)
Jack Straw was the first song Brent played in his first show w/ the Dead, 4/22/79 Spartan Stadium. Bob continued the 'Clive' tweak thru '79, he repeats it on 11/1/79 Nassau Coliseum.
I don't think the Dead ever played for acid, BTW. Owsley built it into the speaker cabinets - it was free. Along with an all-you-can-eat meat buffet ;}