Wasn't Graham's archive not locked down properly given that he died unexpectedly in 1991 in a helicopter crash? Tapes by top acts could have been looted by his own staff -- or he could have given away tapes to employees. Are you suggesting that the opening act for The Doors paid Graham to use his gear and record The Doors, which were headlining? Does that sound logical?
It's unclear what got destroyed in the fire that happened to the archive in the early/mid 80's. Or what tapes "disappeared" while he was still alive.
I've never heard of BGP ever having a permanemt multitrack and the billboard article mentions access to one.
Many of those BGP bills were more MULTIARTIST unique lineups rather than traditional "opener/closer" billings. I also think EVERY multitrack recording from a BGP venue was funded by the label/artist witht he exception of a couple TV short projects/experiments he tried to get off the ground.