You're being quite vague IMHO, secret government project? :-)
You say no human intervention....so who's deciding when to load the new firmware?

Abyway, in theory yes. As long as the PIC can program its own memory you can create a bootloader that reads the .hex file from "anywhere" (USB, SPI, RS232, I2C, parallel bus, whatever using whatever protocol). But remember that it's actually the PIC itself that is loading/programming itself with "information" from the other device - its not the other device that is "programming" the PIC.

There are, for example, bootloaders available which reads the code from a memory card (CF, SD, MMC etc). I don't know of any written in PBP though.

/Henrik.