You should be able to use the 3140 for this application but it depends on what you mean by transparent. There may be a couple of ways to accomplish what you want. You will more than likely have to turn on the 3140's firewall. How many usable public IPs have you been given by the ISP for your network? You will have to assign a public IP on the 3140 WAN and another public IP (from a different subnet) on the 3140 LAN. The next step would be to enable the firewall where you would need a policy-class on the 3140 WAN that will allow the traffic you want to the server.
I hope this helps, but please do not hesitate to let us know if you have any questions.