I love this perspective and partially agree! For me, beeminding scale weight is super great, but only because I’m willing to treat it as an action, not just an outcome. Namely, by sweating [1] and fasting [2] to force myself back on the road. If I weren’t willing to or couldn’t do that then “give yourself a good amount of leeway to account for the variance” would not work at all. Because no amount of conservativeness of rate of weight loss, including a flat road, would prevent me from skating the edge.
For me, Beeminder works by forcing me to take concrete actions on beemergency days. With weight loss I’m beeminding the actions of fasting and sweating, as measured by my scale weight.
I can beemind an outcome but only if I’m also beeminding actions that ensure I always have safety buffer on the outcome graph.
PS: How it plays out for me typically is I wake up off the road so I skip breakfast and if necessary lunch as well. If I’m still above my weight road as dinner approaches then I’ll run up and down the stairs or something until I’m back on the road and can eat dinner. Then I stuff myself and the process repeats the next morning. 
When that gets to be a drag I’ll sometimes, y’know, not stuff myself and sometimes go many days in a row of actually waking up on the road, as has been happening lately:
(The red dots are me weighing in again and again waiting till I’ve breathed/peed/sweated/etc enough to be back on the road.)
Footnotes
[1] Never to the point of feeling dehydrated. In theory you could back yourself into a corner where you’re steadily gaining weight yet eking on to the road every day by dehydrating yourself more and more. But in practice it’s not an issue. Beeminder is forcing you to take action and you’re not so pigheaded that you’ll knowingly take the wrong action. [3] If you were you’d probably want to figure out how to turn “no dehydrating yourself” into a bright-line rule.
[2] Many people may have health reasons not to do this and I’m nervous to recommend it since I’m no expert. But from what I’ve read, both sweating (by which I mean exercising) and fasting seem like non-crazy components of a weight loss strategy.
[3] See our take on why Beeminder is robust to Goodhart’s Law.