I've done this to make single hop beers to highlight flavor differences.
Instead of Mr. Beer fermenters I went with glass carboys. My LHBS had some 3 gallon ones. I found a local water store, the type that sells water for water coolers. I had them sell my a 2.5 gallon glass carboy.
So I did 5 gallon batches, split them into 2.5 gallon batches, and used my 3 and then 2.5 gallon carboys for fermenters.
I liked this better than going with the Mr. Beer thing.
But thats just me.
Here's the 3 gallon one:
I couldn't find a link to the 2.5 gallon one...