I am not sure exactly what you mean. Are you talking about the AP being in a different subnet than the ZD?
Or do you mean if the AP are servicing clients on different L3 network.
In general you only need BG if you are in an L3 environment. If your client (iPad for example) and the Bonjour server (Apple TV for example) are on the same L3 subnet, there is no need to use BG since the MDNS packet will be visible to all device within that same subnet, but if you put Apple TV on one VLAN and your clients on a different VLAN you will need to use BG to allow client to see the MDNS packet from a different VLAN.
If you can provide a specific example it will help to explain it.
But if your question is simply hot to apply it to an AP instead of the ZD here is a copy from the UG on that:
Applying a Bonjour Policy to an AP
Once you have created an AP site Bonjour policy, you will need to designate the
AP that will be responsible for implementing this policy.
To enable Bonjour policy on an AP:
1 Go to Configure > Access Points.
2 Click Edit next to the AP you want to configure.
3 In Bonjour Gateway, enable the check box and select a Bonjour policy that you
created on the Configure > Bonjour Gateway page from the list.
4 Click OK to save your changes.