This is perhaps not quite the same thing, but one feature I did a trial implementation of was "exception points", which lets you conditionally trap any case where the debugger exception callback is invoked. These cases potentially includes software traps, and I already updated the x86 and 68K CPU cores so that INT XXh and $AXXX instructions can be trapped. (The currently-existing gex command behaves similarly, but is a one-shot deal.) A secondary intention behind this proposed feature of mine was to simultaneously retire the special per-instruction debugging hook which is now used exclusively by the rmnimbus driver.