Lines starting with # are treated as comment lines in the registry text files.
I tried that to no avail. I took a registry file that previously built with no complaints and added a line "#this is a comment" somewhere in the middle of it and the build process errors out with "Expected isolation mode or deleted at..."
Hmm, turns out comment lines are not handled correctly by vregtool. Sorry, no workaround. I've filed a bug.
