|While I have no experience with that particular device, I do have experience with other cisco products. This looks like a mid-grade device but should easily be able to do what you want. However, due to my lack of experience with this particular device, I can't advise you beyond saying you might want to:|
1) Find a cisco help forum on the internet, join up and ask about this there
2) Contact cisco support directly (they'll want money)
I would try #1 first if it were me.
Now, in the past I've created port forwards on SOHO level equipment for ftp, ssh and RDC. I haven't tried (or needed to) make a forward for HTTP (port 80) but it should function the same as any other forward.
You shouldn't need to do any more than enter the outward facing IP of your router (external interface) in your web browser and you should be automatically directed to the printer in question.
One last thing, you could put a PC in the LAN in question and set it up to accept RDC connections. Then create a port forward to that PC on the RDC port (3389 if memory serves me) and you could then connect to the PC and from it, any printer in that network. Using the method you're employing, you could only connect to one printer. Using a "printer management PC" you could connect to all of them.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.