^Z because it's otherwise ‘suspend’, so it doesn't conflict with active use of anything else. (Plus ^Z^Z to send ^Z.) I haven't checked, but if multiplexers do

^Z because it's otherwise ‘suspend’, so it doesn't conflict with active use of anything else. (Plus ^Z^Z to send ^Z.) I haven't checked, but if multiplexers don't already use XTMODKEYS to allow more complicated prefixes, they should.