What we have done to single-thread actions.
Created an EICLASS with Max 1
OPER EICLASS CLASS(1)
Class 1: Current=1, Max=1, Active=0
Then added the EICLASS to an Event.
Not sure how that would work with your solution.
You may need to create an APPL/PROC that does the AJ command to RESUB
And add the EICLASS to its event definition.
EVENT ID(<Event ID>) EICLASS(1)