Hi Dan,
I've set up up the IP paging units on a different subnet to the controller multiple times.
They get their configuration via the DHCP options just like the phone.
I'm not sure how familiar you are with the paging unit but the setup guide is pretty in depth.
The basics of it are;
Program options 125, 128 129 and 130 on the DHCP Server
Program the IP Paging Unit on the phone system as a "5010 IP" with a Full License.
Program the MAC address against the extension.
Connect the IP Paging unit to the LAN.
Set the Group Page Accept field to "Yes" in the the IP Paging Unit COS.
Set each telephone that will be paging has the Group Page Allow field set to "Yes".
Assign a Direct Page feature access code e.g. #**
To Page Dial #**EXTNO of unit.