Make the project appealing to designers. For instance let the application have themes/skins and have a marketplace where designers can sell their themes on their own. That way designers dont have do deal with the developers and can work on a complete consistent design. Wordpress and others are doing similar things.

If a designer has created multiple layouts he could also make one for free to get people interested in the other "premium" ones, so its not that every design has to be a payed one.