More useful information (hopefully):

The EXE at each site will not simply serve data, but will do "jobs" for the remote clients.

Also, the pipes between the sites are broadband and secure, but the WAN is nowhere near as fast as the LAN and local computer (obviously eh?). So, the remote sites have to be smart about how much data they send. That part I can handle with good code and so on. That's NOT my worry.

The concern is mostly with getting a connection between the EXE's spread out.

I have considered using a data table to drop requests into and then using a timer on the other end to process the requests and return the data the same way. While this will work and I have worked with systems doing just this sort of thing, I am not in favor of this method due to it being rather slow in response time (sometimes minutes, not just seconds).

What would be really great is to come up with a design where the communication is instant (like a remote method call) or an event trigger.

Perhaps the client EXE can spawn a remote EXE "on-demand"?

Not sure how to design a good solution, so your suggestions are highly prized! :-)