You can do this within Configuration Manager.
Just select the group, right click and choose "Set Sever Property"
That sets the property for all servers within the group.
cool, this achieves the same result but not ideally as I wished for.
The reason for doing it through RBAC simplifies the process of having to filter servers in order to set the property in CM. Imagine you have 100 servers all the same platform/OS and on the same network and differ by the site location. If you haven't got any quick way of updating this property detail in RBAC its going to make it a long manual task in CM to set it up.
You should be able to setup a smart group for most requirements like that, and set the server property for that group.
There are two ways I can see of simplifying this process for you.
One is to create a manual group for each site and add the relevant servers in, which is a lot simpler than setting a property for each server, then set the property for the group. Then you can create a smart group and write blcli scripts using that property.
The other way is to write a blcli script that takes hostnames, and the name & value of the property to set, as parameters and sets the property. I can tell you the blcli commands to use if you want.