Just came across this video of a Bowie concert in June 1972. Your opinions please...at precisely 1:00, there is a glimpse of a bemused young man...could that be Brian? link
This was around the time Roger & Freddie drove up to see Bowie. It's possible Brian was around, although as this chap has not got a notepad jotting down all of Mick Ronson's licks it's probably not him. LOLZ.
Bowie had played at Imperial College a couple of months before. I should imagine that would be a better gig to check the crowd footage for. Although in the film (shot for French TV) no audience is seen.