I think to really make it work well, some sort of dev kit software builder will be needed. You'd give it your own artwork and name and other bits of necessary info, as well as some sort of extra ID info (which the bootloader would need to be able to support). The kit would make a customized copy of Teensy Loader showing all your images and text, and it would only recognize and upload to your boards. It might even bake your HEX file right into the executable and remove the option to open anything else, for an absolutely minimal update application.
Low on Paul's to-do-list, I don't think anyone is in a hurry to see this, would be great though.
One thing I forgot to mention - it would be best if the file chosen to update wasn't human readable for non-opensource codes, hex as mentioned.