Custom Install Options Page based off directory contents...

As sections are called after those pages are shown you can't set settings for the custom page in them. If you don't have a directory selection page then $INSTDIR is set right of the begning of the installer you should have no trouble using it in SetCustomA. If you need to work on $INSTDIR before the page shows do it in .onInit.

that worked, btw...thanks a ton!

I thought .onInit happened before anything else...is it listed somewhere the order in which everything happens?

The exact order is not listed anywhere, but in every callback function docs it's mentioned when it's exectued, sections are executed in the instfiles page, and non-callback functions when called.