GIMP Plugin Registry

Two questions...
I am working on a script that can run for a very long time. I see in
the pdb that there is a cancel callback (gimp-progress-cancel). Can anyone provide examples
of how to use that?
My plugin involves the use of a mask layer parameter. I see some (non
scrip-fu) plugins restrict that type of parameter to one that
"matches" the size of the active layer (an example is in the lighting
render). Any idea if this can be dome in script-fu?

Thanks,

-Rob A>