user1111

I think you might want what is in the render geometry settings lop perhaps? For Karma you can set the holdout mode there for the geometry as None, Matte or Background.

Don't think it's correct but the best I could find was to put it in a add variant block in a for each loop with everything but the thing you want to be a new variant deleted (pack, delete everything else with a removepoint in a wrangle, unpack) using the contextoption("ITERATIONVALUE") to select the correct one.
Load as reference seemed to be required for me to get it to work, don't know USD enough to know why unfortunately. It's nearly procedural with the main value that has to be hand done is the total iteration number.
It's really a mess though, hopefully we get a node that takes packed prims and turns them to variants or from a sop attribute. Would be glad to see a better way than what I've kludged too.

I eventually narrowed it down to the top.py calling a simple python function to get an IP adderss that didn't always choose the right network adapter. I fixed it by putting in hardcoded values to an OS.Environ path, but it would be great if you could set it in the nodes themselves.
I'll also try the new build to see if it got fixed, thanks for the help.