Are there any open task sets available to assess workload using secondary task measures?
Since you mentioned the Stroop specifically, several versions of the Stroop task are available for Inquisit here.

Randall Engle's lab also maintains a set of validated working memory tasks, which are available on request to researchers. They include full and shortened versions of operation-, symmetry-, reading- and rotation-span for E-Prime 2.0.

Assuming I read the question correctly, any of these should satisfy the requirements.