I've just had a dig in the cgen source and noticed this comment in insn.scm:
; Return instruction mnemonic.
; This is computed from the syntax string.
; The mnemonic, as we define it, is everything up to, but not including, the
; first space or '$'.
; FIXME: Rename to syntax-mnemonic, and take a syntax string argument.
and opc-itab.scm:
; Strip the mnemonic part from SYNTAX.
; (ie: everything up to but not including the first space or '$')
; If STRIP-MNEM-OPERANDS?, strip them too.
I'm not sure what I want to do is possible.