Jang,
this is exactly what one of the other Tech Writers here suggested I try (scripting RoboHelp instead of Frame)! 
I just thought it would be easier to script it from Frame, because the new 'Publish' command for Technical Comm Suite 4 is designed to let you do this with a single click in a single dialog ... so I thought the corresponding script would be simple too.
However, it doesn't look like scripting this is a mandatory requirement for the project I'm working on, they're happy for me to generate .CHMs manually and just check them into their build system. So I'll just be doing that for now. I may return to look and see whether it can be scripted later on, once more of the actual content is written