No, sadly, Home Sharing doesn't work across a VPN or even across different subnets to the best of my knowledge. I did try both just experimenting a little... It's either a port that isn't forwarded or some kind of broadcast that doesn't carry across those connections.
Still, as long as you have some kind of computer with you on the same network, it should work fine. Now, playing to both at the same time could be a little tricky, depending on the computer. Still, it buffers the show/movie to the ATV, so unless you start both within a minute of each other, that shouldn't be a problem either.
I'm looking at getting a second one myself in the next few months, so I certainly hope all this works like they say it does.