First let me say that I agree with the posts above that replacing the HU should not void your warrant. However for the sake of information and avoiding work...
1. As far as glove box installations go I thought this one was pretty cool. (also see this)
2. Vertical installation is less than optimal on hard drives, however I don't think I would call it a no no. It simply has limitiations that you have to account for i.e. the obvious visual inconvenience and high probability of shorter HD life. (Please correct me if I'm wrong on this.)
3. And then there's this.