It's not (as far as I'm aware) possible to absolutely allocate a set percentage of empty barrels to booze production. However, there are a few tricks you can try that will help:
Segregate your food stockpiles. A big stockpile that holds only food types that can be used for meal preparation (meat, fish, tallow, syrup, sugar, flour, quarry bush leaves, etc) nice and close to the kitchen area, then smaller stockpiles that only accept either prepared food or drinks next to each dining room -- and the stockpile that holds prepared food has an upper limit of zero barrels.
As of right now it's not possible to have one stockpile feed multiple smaller piles, so your best bet is a logistics chain -- if prepared food stockpile A is the one by the dining room that's closest to the kitchens, prepared food stockpile B (which is further away) takes from stockpile A, and stockpile C (which is even further away) takes from B, and so on. EDIT: As it's now (version 0.34.11) possible to have one stockpile give to multiple others, as well as the ability to take directly from workshops, a good tactic is to dig out and designate a large prepared-food-and-booze-only stockpile very close to the kitchen area that takes directly from the kitchens and stills and gives to all the stockpiles near dining areas.
At the same time, arrange for a stockpile for empty barrels (furniture subcategory, if memory serves) near the area where your stills are located, so that your brewers will only have to walk a minimum distance to get their next barrel.
As of the 0.31.21-5 series, you can craft rock pots in a craftsdwarf's workshop; these function exactly the same as barrels and can hold booze as well. Since rock is almost always ubiquitous, queuing the occasional batch of thirty Craft Rock Pot jobs via the manager interface will go a long, long way...
... For clarification, you can change the kinds of stuff a given stockpile is allowed to hold by hitting
q, then moving the cursor over the stockpile, and hitting
s to adjust its settings. You can alter the number of barrels and bins it is allowed to use from the same menu -- hit the capital
E and capital
C once each to set the number of allowed barrels and bins to zero -- and use the
t key to start hunting for the stockpile it should be allowed to take stuff from.