How to make my custom TaskScheduler the default one
Why not just create your own static method, MyTask.StartNew()? Don't you have to specify the affinity as a parameter anyway?
In any case, the task factory should use the same scheduler you're in when you call StartNew. So if you do that on a task on the proper scheduler, it should work just fine. This really depends on how you handle your processing scheduling etc, but I assume you're going to be context dependent anyway if you're not going to be passing the affinity anywhere.