I think it'll be worth a shot?
'Cos if we build IPSEC tunnels (as outlined above) on the current (I get the embedded aspect) topology, could it not be seen as a risk by potential clients that we demo IPSEC to?
In other words: 2 different business partners connecting to their network, but cannot be separated by individual local encryption domains?
The other option would be to build ("Non-VPN") rules:
Each business partner connects from their public/static IP address to the HQ's NATted IPs for the appropriate/relevant server(s).
(Or...servers are put in the DMZ?)
But, the brief/requirement (in this case) is IPSEC.