I, too, would like to see this. I also try to use a script to keep track of mIRC's current instance, but computer crashes easily yield incorrect results. It would be very helpful if mIRC could tell me which instance it is via an identifier ("$mIRCInstance" perhaps).
Of course, the behavior of such an item would need to be determined: upon opening two instances, and closing the first, then opening a new instance, does the new instance receive a new ID number or revert to the first?
In this case, two identifiers may be appropriate: one to provide a unique ID (which is reset only upon system boot), and another to indicate how many instances were running at the time of mIRC's launch. (This seems somewhat parallel to the function of scid/scon.)
Maybe this isn't mIRC's job, but it would certainly make things easier.
