Comment from an external (ICANN) to this group:
Just a thought: if .XY has an address checker for their country, and .YZ has an address checker for their country, couldn't .XY provide a way for .YZ to check the addresses of registrants in XY and vice versa? Once an interface specification had been defined, operators could then bilaterally engage with each other, with CENTR as a clearing house.
Regards Patrick