I'm under the impression that standard cognitive load manipulations (e.g., remember a 7-digit number while completing some other task) wouldn't work for online subject pools like Mechanical Turk. Are there other cognitive load manipulations that might work online? Time pressure is an obvious one, but anything else? Thanks!
Cognitive Load is very subjective, and typical mental interference tasks (like solving problems while working on another task) aren't used in studies involving Cognitive Load Theory (CLT), except to measure individual differences in Working Memory Capacity before the experiment begins. Most experiments in CLT ensure that all participants' CL is through the roof, to test whether any interventions have an effect on their CL and therefore performance. This is usually done through the complexity of the task itself, i.e. the difficulty of the subject matter specific to the subjects demographics and prior experience combined with a complex problem solving task (usually without mental interference).
So to answer your question, if you're more interested in Working Memory span or capacity, then this article outlines lots of different tasks you can give them. If you're more wanting to look at increasing CL from a CLT perspective, just look at any of the seminal studies by Sweller, Paas, Chandler, Ayers or Kalyuga for how they increased CL during the experiment.