python binding FLAG_REPEAT_BTN seems defective

That's actually the way it's supposed to work - FLAG_REPEAT_BTN causes all button messages to be delivered, but it won't fabricate fictitious messages. Since you only have button reporting turned on, only button state changes will trigger messages, so the flag doesn't do anything. However, if you turn on, say accelerometer reporting as well, each received message will contain accelerometer information as well as button information, and since accelerometer state generally changes much faster than button state, there will be runs of many messages containing identical button state. Without FLAG_REPEAT_BTN, button messages will only be reported when they change, but with it, they'll be reported regardless.

For the functionality you want, check Wiimote.state - it always carries the current button state.