I think just like everything else the boot rom just pulls in the first sector of the disk. I had RX02s on many of the BRL Gateways (my implementation that replaced your MIT Gateway while you were in exile). We put a V6 file system and I must have had a regular V6 boot block on it with a RX02. There might be someone still at BRL who might remember where this stuff is. I certainly don't have it.
BRL PDP-11 kernels had both V6 and V7 file systems in them but I'd have to believe I was booting off a V6 one.
