SelectingWithinGroup

I want to enable ctrl-click to select within group, but I don't see how it can be done. I've delved deep into the arena code and found that if a group is "transparent" then its members can be selected one by one. But how can I do that on ctrl-click? In select-context.c, the handler that checks what button/key was pressed seems to already get a selected item as an argument from the arena code. Will I have to analyze this preselected item on my own, or is there a more elegant way?