A 60Wh battery weighs about 300g. That stage is about 5 hours. 300g seems a pretty small price to pay for a 10W boost, especially if you achieve it by making the bike be under the limit and then adding motor+battery (switchable with a dummy weight, of course) to bring it to spec.

(The motor, of course, would probably weigh more - but it remains the case that you can build a bike that weighs under the minimum.)