On the page Spencer linked I found a promising looking solution.
A simple method is to surround the enchantment table with a 1-block high square of bookshelves with an empty space anywhere on the perimeter:
According to the table on that page you just need to disable between one and seven tables (simply done by putting a torch on them) to get offers from the 20s. I'm going to try this tonight. Once I determine if it works with torches, I might try some fancy sticky piston arrangement to disable some of them at the touch of button.