It seems to me that you need to tunnel your devices. If you have more than one device I would use that to create a tunnel (point to point VPN) to the other side and then go through that. In other words:
PXE <—> DHCP Server w/VPN to Other network and routing <----we shouldn’t care what is here----> Other side of VPN <—> TFTP server
I don’t know why that wouldn’t work. Now doing an “on demand dial vpn”… that I’m not sure of.
On the DHCP server you should have a static route that would go through it to your TFTP server. My understanding as to how it works is that you send a special DHCP request (your PXE boot) to the DHCP server which says “here is an IP now you want to go here” (that is the helper IP for the special PXE request). From there it is only a matter as to HOW to get there. If you have a VPN tunnel established then you should be able to route across that and hit the TFTP server.
Your only issue at that point is more TFTP than anything. I don’t know how it works with FOG but when the TFTP image is brought down if you have issues on your network it will fail and have to start again. What I do not know is how FOG sends the image. if it is TFTP then you have to keep your fingers crossed. If it is more involved and accounts for network congestion/issues during the transfer then it should be fine.