This is an interesting thought. Assuming SD 7.5, you could have a rule on incoming incidents where you check first to see what queue the incident has been assigned to and if that passes then it goes to another rule which is a workflow. You'd then have to create a workflow that looks up the queue, finds its members and then looks at and counts the number of tasks assigned to each. It can then make a mathematical decision on who to assign the incident to.
In 7.1 I think you'd have to do it inside of a routing rule but the same concept would apply. Locate the queue, find the queue members, see if you can count how many assigned tasks per person and do some math to determine where to assign the incident and then do so.
This is all just thought but now I'm curious. In theory it can be done. The trick might be determining the number of tasks assigned per person and then the math. You might just want to determine who has the least and assign it that way. If they are all even then you'd have to determine where to go from there. Interesting for sure. I would also guess there are other ways too.