This is actually a pretty big problem. My cofounder and I run a small video game studio and we worked with our lawyers to figure out how to protect artists as well as the company in a way that seemed fair. What we came up with was a general blanket rule that if an artist's work gets used in any public way (promo materials, game launch) then they can use that work immediately for their public portfolios. In the situation where it's not released we have end dates in the contracts for when they can use their work in their portfolios.