If I recall correctly, the FreeBSD bootblocks will only
boot from the First BSD `slice` on the disk.  This could
be fixed...

In the presence of BIOS partitioning with disklabels, 
sd0a is aliased to sd0sXa, where X is the first slice 
containing a BSD disklable.  (sd0a is really sd0s0a, 
which is why slices atart with number 1 instead of 0).
And yes, you do have an alias problem. :(

I think if you grep through all the system source files,
you will find that a considerable number of them know
about disklabels.  Many of them are likely to have "8"
hard-coded, instead of using the value from a header file.