Thanks for the ideas. I built the mask using a combination of functions in Vegas. I can't remember now whether I just used 8 applications of "cookie cutter", or whether I built the mask and then did the bluescreen thing. The latter is what I did in PG, but my aging brain cells are already hazy on which way I did it for PSB (I know, that's pretty sad).
In any case, I'll go back and have a look - see if I can see anything that sheds any more light.