Hi, this actually doesn't depend on what MPLAB version you are using. It depends on the compiler. Once I made a bootloader for PIC18F4520 in assembly language (MPASM compiler). To put code on specific memory locations in assembly language you need to use "ORG" directive. You should see which equivalent directive your compiler has.
One smart thing to do is when you be done with the bootloader, you can compile it as an object file and include in future projects, so there's no risk to change anything by accident.