I'm having the same problem, and as simonzzz, cannot make it work.
I also tried the jquery field plugin to set the radio button value, but the UI does not refresh.
I really think there should be a method .setValue() on buttonset that allows setting a value programmatically.
Or even better, get the buttonset to listen to value change events and react accordingly.
Otherwise, trying to populate a form from values returned by a server is a nightmare!
And triggering a .click() is not a good idea. There might be submit methods attached to that element...