Start iTunes while holding the option key. iTunes will give you the option of using a different library or creating a new library. Create a new library.
Connect that library to iTunes Match.
When you're back home, start iTunes while holding the option key once more, and choose your old library.
And go to the "iTunes Feedback" menu in iTunes and tell Apple about this. Myself I have a large internal drive to hold my library, but what you have is quite a reasonable use case and should work automatically. Especially for people with an MBA who cannot use huge internal hard drives.