Custom properties for products and line items

Although I can create a custom property for a product, is there a way for this to be accessible to a deal calculation. My scenario is:

- I have added a flag to a product as I want to track certain product types

- I then create a custom property for the deal which is a calculation using SUM of ARR items which have line items with that flag above set. However, I cannot select that flag from the dropdown when I do the COUNT.

Is there a way to do this? Even having NAME in the dropdown would be fine as I could put the text in the name and use that.

Thanks