If you want to replace the original bootloader, assuming it's located in flash, why not create an application to (re)write the bootloader? Use the original bootloader to load the bootloader bootloader, then run that to rewrite the bootloader.

I'd sure stay away from any homemade programmer. The amount of work required to reverse engineer somebody else's programmer, to end up with a likely still unreliable result, just isn't worth it.

Besides, ISP is so much easier.

Dennis