This might do it, although it seems to have far too much functionality:
Looks useful to play with for other things, thanks, even if OTT for this.http://www.ebox-platform.com
It supports OpenVPN, which will do all you want and more.
I was thinking about some of the embedded Linux devices, Tonido Plug, Guru Server, etc, although I'm not sure they'll do what I need (or if they will, then how to do it).