I'm no expert on bootloaders but I'm pretty certain they will only work on the chip for which they were complied. That means, for example, that you could most likely use the same bootloader on an Uno and a breadboard Arduino if they both used an ATMega328P but the Mega uses a different chip (in fact I think there are several chips for different Megas) so I would be highly surprised if you bootlaoder would work. I'm also fairly sure that the IDE won't let you do the upload without over-riding some safeguards.
Also, since the IDE sets the fuses at the same time as burning to bootloader, if you fooled the IDE into using the wrong bootloader you could potentially brick your Mega by setting fuses appropriate for a different chip - not a great plan when the chip is soldered-in!
Summary - I don't think it will work and I wouldn't try!
i dont use ide to upload sketches or bootloaders, i use atmel avr studio , so if i make sure that when i compel it is for the correct atmel chip i hope it will work i will look at the makefile of the standard bottloaders and compare