If /halt in ON KEYDOWN makes the whole client non-responsive, it could be salvaged by using the mouse to get into the scripts editor and uncheck listen/events.
In general, with backward compability issues, I am more worried about new or non-technical users who are using scripts but may not know how to fix issues. Even a technical user would be stumped for a while if they ran mIRC and suddenly found that they were no longer able to type in windows :-]
If /halt is a concern, there could be the addition of a -switch for halt to enable the other keys being halted in that event.
Yes, that may be the best approach.