Anyway, I carted the tripod with me and set the 'Blad up with a 50mm lens (equivalent to roughly 28mm in 35-mm land), composed and pre-focussed. Then I locked up the mirror so as to minimise the shock-wave normally associated with taking a shot with the 'Blad - quiet it is not. Even so, there was still (to my ears) a very audible 'click' when I released the shutter via a long cable release. But I figured if I wasn't actually looking at the subject when they came into shot they might not be too concerned. A couple of times I was on the mobile phone when I tripped the shutter.
The downside of not always looking at the people walking into frame was that I missed a few shots but one or two were OK. Next time I'm going to try framing more 'head-on' shots and see what happens.