By default, all the wireless adapters (SSIDs) and LAN ports are bridged (br0) and treated as a single entity that share the same ethernet segment and IP network. All distinctions between wired and wireless, and how any given client gained access to the network, is lost. And when it comes to the OpenVPN client, you can only split tunnel based on the source IP (at least when using the GUI).
The only way to segregate a given LAN port or wireless SSID for use w/ OpenVPN is to *un*bridge that LAN port or SSID from the default bridge and create a brand new ethernet segment and IP network (VLAN or AP, respectively). And now you can specify that new IP network in policy based routing for routing over the OpenVPN client. IOW, indirectly,
Of course, the downside of doing this is that now you have two separate ethernet/IP networks for all other purposes, and that can be problematic (e.g., network discovery cannot cross ethernet boundaries, at least by default).