(This answers the same question about a different ceiling-mount fixture than the OP's.) The porcelain light fixture I just got at Home Depot has breakout holes for screws on 2.75" centers. A hammer and screwdriver and a little persuasion was all I needed to bust them open. Eye protection would have been smart.
This elaborates onanswers the same question about a different ceiling-mount fixture than the OP's. It gives one example of the first sentence of Michael Karas' answer. (II googled my way here looking for instructions on opening those inner holes. Turns out brute force works.)