Valid Intersections Select Prior Months from Substitution Variable

Summary

Need to set valid intersections for prior (or following) months based off a substitution variable.

Content

We have a substitution variable that controls forms and rules for the current month in our application. Until now, we have used scenario periods to lock the prior months and valid intersections to control writable accounts. There is a new requirement to leave a few selected accounts open for prior months.  Ideally, I would use some syntax like done in rules (Jan:&CurrMth) or left siblings.  However, for valid intersection member selection, the rules syntax is not available.  And left siblings only sets for siblings of the same parent.