Why would it only play on a computer? Surely if I can play a movie from USB on my blu-ray player then there should be a way to have a menu with it?
No, you can't.
1. Not every blu-ray player has a USB port.
2. Of players with USB ports, not all of them can play movies from a thumb drive.
3. The ones that do play movies play only specific codecs, none of which are interactive. Just a movie.
4. The ones that do play movies all use a proprietary operating system. Even if you could hack a card (so to speak) to pop up on one brand/model of BluRay player, it's not going to work on other brands/models. It's relatively simple software running on the devices. It's not as simple as writing a PC and a Mac version of the pop up menu software, there's hundreds, if not thousands of models.
Now, IF you had to do something along those lines, you'd be better off first writing an "App" for it. Many blu-ray players can download apps (like netflic and hulu) and run those. That app could then recognize the USB stick, but seriously, at that point you'd be better off streaming the entire thing. Even then, like netflix and hulu, you'd have to write different versions of the app for every brand/model/operating system out there and would need a huge team to do it. THEN, you'de need to be recognizable enough for them to pick you up and distribute that app.
The size of the cards is a great point. The Blu-Rays you buy of Hollywood movies hold 50gb. The reason they're cheap is because they're one time write, slow to write to (unless you're stamping, which is expensive unless you're making thousands) and made out of cheap materials. Instead of paying $0.05-$0.50 a disc you'll be paying $30-60 per thumb drive.
Additional future content is a cool idea. Sell them a Blu-Ray with an extra spot in the case then mail them a supplemental disc in the future. It will cost SOOO much less, and will work!