>The system knows nothing about MBR partitions. It just knows that
>there are these "partition" things listed in the BSD disk label that
>list sector ranges corresponding to partition a, b, c, etc. c is
>traditionally the full MBR partition and d the full disk -- don't
>touch those -- but you could easily point, say, h, at any range on the 
>disk you want, including another full MBR partition.

Small pedant point: 

sys/arch/i386/i386/disksubr.c knows about MBRs, cuz it uses MBRs and
dos partition IDs in order to decide where to find the BSD disklabel
and read it.  Otherwise, yup.