>   The explaination is simple (and probably not documented anywhere):
> 
> 	loadbsd expects the value for 'I' to be hexadecimal.
> 
> 	bootblock expects the value to be a C-style number.

Argh.  Changing it worked.  I recompiled the bootblock to include I 0xff and all 
is well.

Thanks. =)

[...and, you're right, I can't find it this documented anywhere either...]