I'm not sure how to access sheetset stuff via VBA as I've never needed to do that.
But I did once need a simple listing of the files in a sheet set and was able to get that by selecting "Publish Dialog Box" from the Sheet Set Manager. Inside the Publish Dialog box you can save that list as a *.DSD file, which is a plain text file that lists DWG, layout name, etc.. That file is in a format that should be fairly easy to parse programatically.