The incentive for people to contribute to a closed source project isn't all that much. Remember that open source isn't a gift by your company to the public, it is an offer of trade -- you let the public have the source, the public provides you with feedback (bug fixes, enhancements, etc.) and gets its suggestions provided back to it. It's a circle.
What you are suggesting sounds like you want the benefit of that deal, while negating the benefit for those who are doing work for you. Psychologically, it's a hard sell to say to someone -- "mow my lawn for me and I'll sell you a lemonade afterward at full price --- um yeah, I'd also sell you the lemonade at full price if you don't mow my lawn." You aren't going to get many takers for that deal, and the ones who do take it will have questionable motives (scoping out the property) or will just be naive and gullible (not a great foundation to build upon).