To me, the simplest solution that comes to mind is to grade on a curve at the end of the course, based on the quality of the work (or some other subjective measure - I'm not familiar with university art courses, but I assume there's some kind of widely-accepted grading methodology). That is to say - tell the students before the course they will be graded on quality or quantity depending on the group, but grade each student on a curve relative to their own section for their permanent grade once the course has finished. This would obviously still require lying to the students (at the very least by omission, arguably), however.