Looking into encrypted USB bootloaders too, it seems that diolan's is the only one available (free or not, though diolan has the big advantage of being free!). Though, it uses the extended PIC18 instruction set which is not compatible with PBP compiled code... There's probably no chance to make it work as is.

Microchip's HID bootloader does work with the standard instruction set, but does not support encryption.

Any idea if there's such thing available?