want bootstrap to force page/view if missing parameters

I have a project which requires a parameter, "group", to be set to one of two possible values.  If it is not set or set to an invalid value, I want to display a page that provides links with the valid choices.  Since all controllers will branch based on the group, I want to do my checking for its existence in the bootstrap.

My goal is to force the bootstrap to go to choose/index if the group param is missing or invalid, *regardless of the current controller/action*.  I've played with the router and request a little, but I'm not having any luck.  setDefaultActionName() worked, but only for the default path (index/index).

Can anyone provide some guidance, a solution, or maybe a better approach?

TIA