The only problem that I could see is the amount of
CO2 being produced. In a 2L bottle there isn't enough wort (sugar water mixture) to create copious amounts of
CO2. If you are planning on a 5 gallon batch (average homebrew size), a little air line would cause the top of the bucket to become unsealed. Resulting in a nice mess (nightmares of first home brew attempts).
Now looking at your suggestion of doing a half batch size is the amount of head room you have in the bucket. You'll have a bunch of
CO2 just sitting in the bucket (heavier than air) that wouldn't go into the reactor. I think the 2L bottle is the perfect size container as you have a minimal headspace to
CO2 produced, as well as the ability to vent the pressures created by the yeast with the air line.
If you are planning on just adding yeast, you'll have to remove the watery/alcohol mixture then top of with fresh wort (sugar/water). Plus maybe use a champagne yeast. 18% alcohol tolerance
But that is my 2 cents.
Just because I am working at Petco does not mean I am a sell out, they are still an evil empire, that I am trying to make better. 
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