Q:
Guest-Visits on foreign Servers
A:
It’ll be ready when it’s ready.
But also don’t be surprised if they scrap the idea entirely. I remember a big selling point of Counter-Strike: GO was supposed to be cross platform gameplay so that 360, PS3 and PC players were all on the same servers, they touted it for over a year and then scrapped it near release. Companies change ideas all the time, Anet may realize the guesting feature is too cumbersome for their servers or something.
But for now, as far as I know, the idea is still planned. Just have no idea when.