Jtag opens up the serial bus on the processor. you can see any bit of code that passes across the processor die. And no there is no numbering. You can see system calls, and the system stack of current operations, but there is nothing telling you what is what unless you look to see where things are defined.
Ergo, finding the bootloader checksum code is not like (oh hey, there it is).
It would be more like making a system dump of the startup. which is all the bootup information. Then you wade through the dump file like wading through a pool of shit looking for a single needle.
It would all depend on the situation. like if you had a metal detector.