The question was just raised by a client ... and I think I know the answer but let me ask anyway ... as to whether-or-not we could use a designated computer on an internal network as an "OpenVPN-based routing appliance" (so to speak) which would allow anyone on that internal network to use it to get to "the other side."

In other words, this computer (which is located on the internal network, has access to the Internet, and, you may assume, is referenced by necessary fixed-routes and DNATs set up on the necessary routers and firewalls ...) connects to the remote machine, then provides (I think ...) "gateway" connections to the remote side from any local address on the internal network, without requiring any special setup, software, or configuration on any of those machines. Instead, OpenVPN acts exactly as any physical router would.

OpenVPN uses transport networks. When you have an established connection simply point your default route at the other's side transport net IP address and give your local clients that openvpn box as the default gateway.
Probably missing something (masquerading etc.) but the general idea is valid.

HTH
Upuetz