No way they'd lock in a deal... for a bunch of reasons including the fact the ARU have to rotate test matches around the various rugby centres every year and the Coliseum will be up and running by the time they're due back in Perth, all things being equal.
That doesn't belong to footy, by the way. Col's signed up for a massive, big shiny new Iseum without any anchor tenants so whoever takes over running the new ground will be trying hard to get other codes through the door.
That means games throughout July and August usually... the Southern Hemisphere nations play their internationals during those months most years, unless there's a World Cup on, skewing the calendar. Long story short, International Rugby Tests shouldn't interrupt finals' fixturing in Perth for AFL.