For example, if you (someone without visa constraints) currently live in London, would Google/FB let you interview in the local office if you intend to relocate back to the US if you get the offer? Any idea of companies that generally do/don't allow this? Seems like it would be a lot easier than flying across the world to the HQ of every company you're interviewing with.
They will pick the office closest to you (to keep the cost modest).
Are you also matched with a local recruiter or one at the location you'd work in?