it's physically possible to surface mount the tweeters in that location on an R32. So that's settled - tweets are going there.
The sub box won't be the end of the world if I go the wrong way - it'll be easy to swap in another.
One practical concern though - how to mount the box so it doesn't go flying on a hard launch? The only thing I've been able to think of so far is to somehow tie or attach it to the rear strut bar.