About once every 18-24 months I get rebooked on American, it is rarely offered but when "suggested" there's usually not much friction when the issue is UA controllable (maintenance, crew, etc.) -- I haven't really tried for an uncontrollable delay.
About two weeks ago I had an experience (see separate thread) where part of it Involved me asking an agent for a supervisor, declining and the telling me there were no UA options and "we can't book on American". Being in my last nerve I may have not so delicately said "yes, you can" to which the response was "oh, you do need a supervisor, don't you?" The supervisor told a different agent to book me on AA if that was the best option (and even authorized cab fare since AA would probably be a different airport).
I've since confirmed at a handful of UA Clubs that rebooking OAL is definitely an option and they couldn't speculate as to why I got the initial answer except for the agent possibly not realizing that it was a controllable delay, as that is kind of the answer for most uncontrollable issues.