Re: could I use the PIC24FJ256GB110 as on-board programmer/debugger IC ?
Programmer/debuggers such as the PicKit3 use various Microchip MCUs to do the programming and interactions with the debugging functions of the IDE and the debugging kernel code so there is no technical reason you can't do the same.
Perhaps using a PIC24 device (of any sort) is a bit of an overkill but, gain, it should work.
The programming protocol is certainly published and the debugging protocol (for use with the debugging kernel used by Microchip) may well be published; ditto the protocol used to communicate between the IDE and the PicKit.
I think I've seen the code for the PicKit2 somewhere as well.
If you can find the appropriate code for whatever chip you want to use then go for it. If not then you can always try to write your own code (given the protocols mentioned above).
I know that there is code for a bootloader generated by MCC for whatever the target MCU is but bootloaders generally don't do debugging (well, if at all).
Check around the internet and you may find useful information. Others here may help you further.