So you DON'T want to use either a computer, SPI or RS-232, is that correct? Or do you mean that you DO want to use SPI or RS-232 just NOT from a computer?

This isn't actually bootloading but the result, I think, is what you're after: One way to do it is to get a PICKit3 and use its Programmer-To-Go feature. You download the firmware file into the PICKit3, in the field you connect the PICKit3 to the programming header on your board and press the button - done. You do need to supply power the PICKit3 thru its USB connector but its just power, no data.

/Henrik