We have a number of locations without a domain controller. For these locations we just grouped their IP addresses into the same site as the Hub site and pointed their local DNS to that Hub Site.
After doing some more research this is clearly not best practice for either AD or DFSR. The following article gives an excellent workflow of how a DC works out whether to act for a site. In order for this calculation to work you should have the sites without DC's added with their IP ranges and IP Site connectors setup with relevant weighting metrics to specify which is the preferred AD site to use for logins.
Automatic Site Coverage in Active Directory Domain Services - John Policelli's Blog: