Restrict BSC life event to current plan/option + Waive: config or fast formula?

Summary:
We have a medical plan type with 4 plans (Plan A, Plan B, Plan C, and a Waive plan), each of Plan A/B/C carrying 4 options (Employee Only, Employee + Spouse, Employee + Children, Employee + Family).

Requirement:
This is processed as an administrator activity in Benefits Service Center (not employee self-service). When we process a specific life event (New Medicare/Medical Eligibility) for an employee, the electable choices presented to the administrator should be restricted to only:

1. The plan and option the employee is currently enrolled in, and
2. The Waive plan.

All other plan/option combinations in the plan type should not appear as electable choices, regardless of which plan/option the employee currently holds.