Ideally, I was looking for a way to *only* allow one of the Installation Types. In other words, disallow the user from selecting more than the sections associated with that InstType. In my case, I didn't want to allow the user to select both "Eval Server" and "Stage Server," it needed to be one or the other.
After looking at the source code, I couldn't find a way to disable the "Custom" InstType, so I created a workaround that involved inserting some custom C++ code into main.c of exehead and writing a conditional NSI file. Unfortunately, it isn't generic enough code to submit to Jason for possible inclusion in the main NSIS code path. I'll need to think of a better approach for that.
p.s. FYI, y'all can add Kenamea (www.kenamea.com) to your list of companies that is using NSIS.