"Back-Filling" Holding Returns with Index Returns

Currently our only option to handle the scenario when a security doesn't have "enough" price history is to re-weight the other holdings in the portfolio - what we do in our "Oldest Security" starting point. A different solution to this problem is inserting a selected index's returns in replace of the security that hasn't been around since the portfolio's inception date.

This is used in Morningstar and a pretty frequent Excel request by our clients. I think it would make sense to introduce this feature when we introduce the ability to enter a custom start date for a portfolio.