This is because there's bitwise ORing involved in the creation  
of the final opcode, which makes z80a's approach impractical.

This makes labelling a bit different too. Instead of expecting  
label words after the naked branching op, we rather have label
words expecting branching wordref as an argument. Examples:

L2 ' BRTS FLBL! ( branch forward to L2 )
L1 ' RJMP LBL, ( branch backward to L1 )