I have a working concept that requires one small hack for Pixelpost v1.7.1
The addon allows you to choose which templates are publicly viewable for selection via an admin settings panel.
No template will be selectable if it is set as private or does not exist.
Your viewers can switch the template by drop-down or by URL string (?template=simple)
Contact me via PM if you wish to test. I'll only be handing out a handful so first come first serve!