Yes, it's probably the Hackford film. There's a chance it could be the 1973 San Francisco reading, but I'll put my money on the Hackford film. Those two readings are the only ones I recall having been filmed by 1975.
maybe they weren't that picky with their choice of words, but I'd think a "videotape reading" is most definitely not a documentary (even if it contains excerpts from readings), which would rule out Hackford's docu. Anyway, as far as I know, it was him who filmed the SF-reading for his docu, right? This would make him the source of the material in any way.
I can't imagine the Bellevue-reading to be publicly broadcast then - given the quality.
I thought the same thing at first, but in 1975 there are dozens of short newspaper mentions like the one above, and they are all for screenings of the Hackford film (though usually on TV, not in a gallery).
I think considering that this one is from the same time period, it could be Hackford. It seems like a videotaped reading we haven't seen would be mentioned somewhere else. But who knows. The San Francisco reading where Michelene bum rushed the podium wasn't widely known, and I have to assume that he was videotaped at many of his readings, since it was a relatively new technology, at least the smaller, less expensive formats of 3/4" U-Matic cassettes and 1" reel tapes. Those were new and cool around the time he was doing a lot of readings.
Which is a long-winded way of saying there must more out there, somewhere. Though most of those tapes are probably in landfills by now.
And I would bet that there is audio of almost every one of his 1970s readings, even if they are just audience cassettes. They exist. Whether they'll ever be "found" is another question.