Let me explain.
Boot loading requires two pieces of code (chunk) one at either end of the chosen communication medium [example PC->RS232 -> RS232+PIC24chip]. (example: Full duplex RS232, half duplex RS485, half duplex SPI, even half duplex Ethernet)
Each "chunk" of code needs to deal with several issues at either end of the communications channel. One of these is the actual hardware interface which will change depending on the type of comms channel to be used.
However all the other issues within each chunk should remain the same. So it would be entirely possible to create a boot loader with the hardware parts missing so that they can be implemented as required in a separate module. Of course it would be useful to have an example to work from; say RS323 interface, half duplex.
Many boot loaders for various chips tend to embedded the comms hardware handler and so it is difficult to extricate the handler and add another comms hardware handler. However if you start off with the idea of a separate hardware dependant module it should be relatively easy to add the hardware module - even for two tin cans and a length of wet string
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