Depending on how you define "stock ROMs" I don't think that such a beast is possible. The stock 64K ROMs do not contain drivers or memory mapping for the SCSI port. So any upgrade which adds SCSI must make some addition to the firmware of the machine.
Now, I was talking with a programmer who did low level Mac stuff wayyyyyy back when, and he said there was a little address space up at the top of the memory map which one could use to hijack the program execution at start up. Apparently this little area gets executed, but usually all it does is send execution back down to the lower addresses. Or something like that. This was a few years ago. The point was, there is a hook in the boot code of the original Mac that a developer can use to gain control of the boot process and add his own code.
So, one method of "modifying" the ROM would be to map that address to a PROM on the expansion card and after loading the necessary code turn execution back over to the 64K ROM.
However, from a software point of view, this would look just like a modified 64K ROM such as the LEVCO uses. The only difference is that one stores the original and modified ROM on one set of chips and the other stores the original code on a different set of chips from the modified code.