Tim,

Many thanks for your thoughts. The collapse has to be pretty much instant,
so any release of pressure on an inflatable dummy would need to take
immediate effect or nearly so.

I'd like to avoid electronics if possible, in favour of keeping things
simple.

Suspension from above is an attractive thought, though unfortunately it's
the hardest of those options to set up, at least as things stand at the
moment. It might be possible though to place the figure in an archway or
similar, but the supporting wire (or whatever) would need to be
undetectable, both when taut (which is fairly easy to achieve) but also when
released (which is harder).

I keep coming back to the lazy-tongs, collapsible pole idea, though the
former would have to be strong and sturdy enough not only to support the
figure but to do so without swaying, and the latter would be pretty tricky
to construct and control, unless there's an existing commercial product
which could be pressed into use. Is there, I wonder?