It would add "Trigger this activity based on a Session Custom Variable," which would reveal a dropdown next to it that would have the Session Custom Variable Dates (similar to how "Trigger this activity how many days before the Session's primary date" makes a box where you enter a digit).

I don't just want my Custom Variables to be data holders... I want them to be actionable. For example, I have "Sale End Date" as a Session Custom variable - I want to be able to use it in a Trigger.