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.