No you cannot. Basically Biergarten is like any of the character buffets. Normally the only exceptions to full price are if the person is on a feeding tube or ill and one person in the group does not eat, or a child that does not eat at all, it is generally up to the server and/or manager to either charge nothing at all or a $5.00 entertainement fee.
Also you could not order a soda only as only alcoholic beverages are specially priced, everything else is included in the price of the buffet.
If you went in as a single you might get away with it. However, you must also realize that all tables are designed for eight people and you would be seated with one or two families.