Refundable when and by what criteria?
You need to lock up 1 million BTS to register your name as one that can be voted on as a delegate.
It's refundable as soon as you unregister as a delegate.
You already need to pay 2 weeks of earnings or ~60k BTS to register a 100% delegate, that seems sufficient to me.
I don't see how sacrificing two weeks of pay really changes anyone's behavior. People just work hard for two weeks to break even. Then they are incentivized only to do the bare minimum not to get voted out.
If you must have 1 million BTS locked on the blockchain when you are a delegate, then your incentives match the incentives of all BTS holders. Your 1 million BTS is always at risk losing 30 or 50% of its value in CNY or USD if you are not performing well and adding value. This is much better than incentivizing people to work for two weeks to break even.