Took me a few days but I managed to resolve my own issue (without breaking the normal of how WF works).
What I ended up doing is, using reflection, iterated over the child's properties and created a LinkedList of evaluation expressions (using VisualBasicValue) of each of its arguments, in the CacheMetadata method. Then in the execution phase, I scheduled the execution of the first evaluation. In its callback I iterate over the remaining evaluations, scheduling the execution of the next evaluations, adding the result to a dictionary, until its done.
Finally, if there are no more evaluations to schedule, I schedule a final activity that takes the dictionary as its argument, and can do whatever it wants with it. Upon its own, it optionally returns the final result to the container's OutArgument.
What I previously failed to understand, is that even though the scheduling occurs after the instantiating activity's execution, the callback runs before control is returned to the host workflow application, and in that space I could work.