Unfortunately, there's more to the internal identifier than simply your Ethernet MAC address, and trying to spoof it with the other account would potentially cause other problems as well in terms of your home Internet routing.
The 90-day restriction seems to be a limitation that Apple was forced to put in place as a result of the licensing issues around iTunes Match -- concessions obviously had to be made to the music industry for a service that would effectively allow users to effectively match tracks from any source with equivalent, higher-quality tracks from the iTunes Store.
The only way to work around it would be to use an application such as VMware to setup another "virtual" Windows machine. A copy of iTunes installed in a VMware partition would be seen as an entirely different computer by iTunes, and would therefore be a way of getting around the limitation, since you would technically
be running that copy of iTunes on a different (albeit virtualized) PC.
Note that "Microsoft's Windows XP Mode" (Download Windows XP Mode
) may also provide a solution for you, since it uses Microsoft Virtual PC to create a Windows XP virtual machine on your Windows 7 computer.