With the free strategy, is it possible to create a sort-of checkpoint that forces tasks to wait until all hosts have caught up?
Additionally, can a block specify a strategy to be used within?
No, strategy is something that only works at play level. If you wan't
to make a break for hosts to catch up, insert a play with linear
strategy in the middle and create a task there.
Then a 3rd continuation play with free again, can be added.