I suspect you will have issues with the RPi I fear it may not be sophisticated enough to fool the network all it would be doing is mimicking an unmanaged hub. It does sound like your Mac is effectively running an Intranet where it takes anything from a second device and packages it as it's own.
The RPi should be capable of the same but actually how to set this up will take some research. Your MAC will also have a lot more processing resources to throw at the problem.
Do the rules explicitly restrict "Wireless Routers" I presume If it is specifically restricted to routers then you could connect your RPi to the port and use a wifi n150 to communicate with equipment in your room.
It does sound like it's an effort to restrict bandwidth use.
While your efforts to circumnavigate this restriction could be seen as tying to subvert the policy I would consider the policy a little short sighted on their part unfortunately my support is of little help to you.
I would recommend caution in your experiments. University politics can end up being convoluted at best.
I would however investigate one of these http://www.maplin.co.uk/gigabit-usb-2.0 ... tor-350301
if you could borrow one for experimentation connect your RPi to the wall port with the ethernet and your unmanaged hub to this USB Ethernet adaptor. I suspect you are going to get a bandwidth restricted by the USB and your probably going to need a USB Hub with a fairly meaty power supply although you could drop the load if you were planning on running the RPi headless (No keyboard or mouse using ssh).
Here I would see the RPi acting as a filter between the wall port and your unmanaged hub as the Uni network would only see the one MAC address provided the RPi has some sort of firewall.
While there has to be a solution if you are able to do this with the MAC a description of your MAC solution could be useful. It will probably be gibberish to me but we may get someone who can help (I am Windows XP Linux).
I have to emphasise caution again the University may see this as attempted breach of policy.