Export PNG with alpha channel/mask

[Ben Rubinstein]

I want to compose an image using a bunch of LC objects, then export a PNG, 
with an alpha channel containing a mask (ideally > 1 bit, but that part's not 
critical).

The documentation for export snapshot is not very clear. I've tried things 
like exporting a transparent group containing some objects; the PNG was just 
filled with black where there were no objects.

There is an option "with mask xx", but it appears that this is only for PBM 
format - and in any case I can't make it work.

Is there any way to do this?

TIA

Ben