Because connectors will differ between different bike systems your DIY'ers might have to do some modification. There is no real standard AFAIK.
You really want to tap in between the battery output and before that feeds to the controller. I wouldn't get caught up with plugging into the controller. Just think about the battery as that is your source of power. Therefore you want to connect to that somehow. The plug types will vary depending on the battery and plugs on each individual bike etc.