With flow meter: Pause currently running schedule for flow greater than "x". This one would take some work.
I think obmd1 has suggested the best answer - IMO the key point is not showering or whatever, rather it is that "something" is trying to use the water during a scheduled irrigation event...the Iro shouldn't care about the reason (e.g. showering, laundry, dishwasher, etc.) Whatever the reason, if the water pressure is lower than some preset limit, it probably should postpone starting a watering session (and maybe even interrupt one in progress?).
I wonder...would something like a well pump pressure switch work? Not sure what the Iro has for external interfaces (like I said, I'm a noob).
Besides, with my two aforementioned teenage daughters, the detect-movement-in-bathroom approach would wipe my lawn out .... one word: makeup.