Hi Ross,
What did you mean for policies at the site collection level? I am wondering whether you mean that you create information management policies at site content types.
I did a testing as:
- Create a custom site column called "CustomDate"
- Open the site content type "Document", then add columns from existing site columns.
- Then select the CustomDate, and under "update all content types inheriting from this type?", select No.
- Click Information management policy settings, and enable retention
- When adding a retention stage, I could select CustomDate, like the screenshot below:

Please test again as the above, or create a custom content type and add a Date site column, test.
If I misunderstand your meaning, please feel free to reply.
Best Regards,