I didn't put the GPL in as part of the installer, because it's not necessary for the user to agree to it (or even for the user to see it). If you want to make sure people see that it's GPL, I think the best way would be to attach it as the EULA in the installer. That's very easy to do and well-supported.

So please, do it. If I didn't care about license, I would have released Sweet Home 3D in public domain.

Most of our users won't ever browse to the folder but will just use the app straight from the PortableApps.com Platform.

If I follow your way of thinking until the end, then who cares about licenses? After all, it's free, it works, that's all what counts! Sorry but this is certainly not the best way to promote the spirit of free software and recognize contributors work.
Some people care about licenses, and for that people, please give them an easier access to license by moving license texts at root of installation directory. winPenPack accepted that change, I don't see why you wouldn't.

We'd like this to be as official as possible with you; our standard naming convention would have "Sweet Home 3D Portable" or "Sweet Home 3D, Portable Edition" - perhaps that second one appeals more?

I already explained that I give to "portable" a meaning that covers more than just moving an application from one Windows computer to an other.
So please, keep the name "Sweet Home 3D" unchanged since it's the same application, and show more clearly it's PortableApps distribution by storing your distribution in a folder named "PortableApps-SweetHome3D" by default (place your platform complete name first).

For your information, portability of Sweet Home 3D across different platforms is ready for the coming version 3.0 (java.util.prefs aren't used anymore). I just have to decide whether I'll include 3 JREs with it or not.