I've found a pretty nice tutorial with instructions to set up the diolan bootloader, with extra infos added to diolan's official website (which are pretty straightforward already). There's apparently a bug in EEPROM bootloader triggering with a proposed fix for it.

So far I'm having a hard time running the code. I ported it to 18F4550 by modifying all the references to the 18F4455, and added the missing config bits for the extra code space. All the define for the config (and especially the oscillator) should be ok, as I made a little LED blink program in PBP using the same config and which does work.

The bootloader compiles in MPLAB, I programmed it and inserted by hand the HEX code of my blinking led program at address $0800. The bootloader does behave as expected, after reset, if the loader button is pressed, the LED never blinks which tells that the bootloader is engaged. However it does not shows on the USB peripherials (and I declared existing VID/PID in the config). Bootloader LED never light up too...

Time for more troubleshooting!