As I understand the service is part of the site, thus you can read it's configuration. If you created a ConfigurationSection (see:
http://msdn.microsoft.com/en-us/library/2tw134k3.aspx[
^]), you can simply use it from any part of the site, by opening the root config (or the one you need).
It is up to you to add an attribute that maps a config branch of the section to a site by its DNS name, but it is somehow confusing, if you want to use the same service under many sites. Is the same virtual path added to all sites - or how do you link exactly the service files to many sites?