The only thing that hurts numbers in open at the moment is other players. If you want to encourage people in to Open play, deal with those players. Carrots only last as long as someone doesn't rip it from your teeth and spank your botty.
Let in game systems 'deal' with criminal players - not overly restrictive game mechanics that forbid certain play-styles. For example, if PvP bounty hunting tools were better, there would be less player-killers (or they would at least be more controlled). If you take options away from players, sales will fall and everyone loses as the devs will get starved of money.
I find it interesting that you say it would "hurt" open..please explain how it would hurt open and why open would need bonuses
The playerbase gets further split into different modes, which results in open play feeling emptier than ever.
With a periodically awarded bonus for time in open, players (especially traders) will be willing to take the risk and go back into open. How I see this bonus working is; every 25 hours of time in open, the player is awarded a 10% credit bonus of the money they earned in that 25 hour block.