A few years ago, Capital Bikeshare tried a "Reverse Riders Rewards" program that rewarded CaBi members for moving bikes from “typically full” stations to “typically empty” stations during the weekday morning rush hours as a way to supplement their normal rebalancing efforts. The program didn't last long because, as I recall, it mostly rewarded members for doing things they were already doing.
What they should do is add a "Rebalance" button to Spotcycle that lets users help rebalance the system. Basically, when you push the button, the app will use your location to identify any nearby stations that are full/nearly full or empty nearly/empty and assign you stations to move a bike between if you're close enough to some.
And then that's it.
I'm sure some people would rebalance bikes, out of general altruism. Just like people create data visualizations and hack apps for free. It only takes a few. It likely wouldn't cost anything extra and any additional rebalancing just lowers cost.
Or make a game out of it. Top "citizen rebalancer" every quarter gets a gold member key and public recognition. Or a cupcake. Or socks. Or a free membership. Or maybe you need to do a certain number in a year to get various rewards. Or do it as a lottery. Etc...
But by making it an assignment, tied to you individually, it should cause people to do actual additional rebalancing. You could even tax people 1/10th of a rebalance if they ask for an assignment and refuse it, so as to keep people from just being opportunistic about it ("Let's see if the trip I going to do anyway will earn me a rebalance").
Anyway, I still think there is a lot of opportunity to use CaBi members to improve the rebalancing, they just have to try and tinker with it.