Before the premium plan revamp of 2016 there was an option for a kind of “ultra-premium” plan called Beekeeper, where someone at Beeminder would serve as your goal-setting life coach. That probably served the purpose you suggest but there wasn’t enough demand for it to keep it around.
The “Weaselproof me” checkbox goes a little ways towards implementing it, but like you say, determined weasels can always lie.
@chelsea wrote about how it gets easier and easier to lie in the great Weasel Heart-to-Heart blog post. That might offer some solutions as well.
Finally, there is a bit of community policing going on in the “Let’s nag each other into greatness” thread.