Hi Jacob,
Could you let me know your demand in more detail about "using a custom field in the account entity to create a contact based marketing list" ?
As far as I know, you can select entity(Account, Contact, Lead) for Targeted At when you creating a Marketing List.

If you want to create a custom querying field, simply go to Settings > Customizations > Customize the System, select any entity you want, then open its Fields to add new one with Data Type in Lookup.
It allows setting a reference to a single record of a specific type of entity.

Were you planning to create a Lookup field in an Account form and query its associated Contacts, then select some contacts to create a Marketing List?
Regards,
Clofly