Assuming that the APs can contact the DHCP server at the head office over IP, you can run a DHCP relay agent on the AP. To enable this, create a layer 3 interface (VLAN) with the same VLAN ID as the wireless clients' VLAN, configure a static IP there for the AP and configure the head office's DHCP IP address under the 'Relay Agent' option.
If your branch router supports DHCP relay you could run a relay agent there instead (assuming the APs are simply bridging and do not have NAT / routing enabled for the wireless clients).
You can also configure more advanced options such as setting the Circuit ID and Remote ID in DHCP option 82 that will help the head office DHCP server assign an IP from the correct pool. See this knowledgebase article.