Billing Email per Site

We've got customers who like to split their billing per site, and additionally have accounts email addresses set up for each billing location, each location is responsible for paying it's own invoices.

We would like to be able to assign a contact to a specific site for billing emails.

Hopefully it's as simple as just getting a button in the site tab of the company that allows you to pick an alternate billing contact and then select from the list that references the Contacts tab from the Company.

Setting them up as separate companies doesn't work, because the Office365 contact sync is only with one company.