Wireless – Which address should I choose to send a multicast message on an wireles ad-hoc meshed network

The short answer is yes, it is not only possible, but happens all the time. Examples of this are CTS-to-self, beacons (sent by APs), probe requests, probe responses and a number of other management types of frames.

However, it seems that isn't exactly what you mean and are looking for a way to transmit data from one device to another without any sort of established relationship. The only software I know of that transmits data in this fashion are tools used when penetration testing wireless security.

Generally, these work by replaying data captures back into the air. Instead of replaying captured traffic, you could craft your own 802.11 frames into a "capture" file and replay any data you wanted. Your second station in monitor mode could then capture the traffic out of the air.

Do keep in mind that this would be a very rudimentary form of wireless communication and does not provide any mechanism for retries/retransmissions (recovery from collisions or other interference) or a means of encryption (unless you encrypt the data before creating your frames).

Switches are not like hubs. Hubs repeat every frame out every port. Switches selectively send the layer-2 frames to the port where the receiving host is connected.

Switches are layer-2 devices, so they only look at the layer-2 frames, not the layer-3 (IP, etc.) packets, or the layer-4 (TCP, UDP, etc.) segments. Switches learn the MAC addresses which are connected on each port from the traffic through each port, and they only send frames destined for a MAC address on a port to that port. Switches will flood broadcast frames to every port (but that's the point of a broadcast, it is destined to every host on the broadcast domain), as well as unicasts which are not yet in its MAC address table.

The HPE 1920-8G-PoE+ is also a layer-3 switch. Layer-3 is where you will find IP packets, but layer-3 routing only sends IP packets to the ports which have the network of the destination IP address.

You didn't include any configuration, but I seriously doubt you have a problem with traffic which is not destined to a host connected to your WAPs traveling to your WAPs.

Edit:

Based on your comment about this being multicast, you need to use IGMP snooping to deal with your problem. Multicast frames, like broadcast frames, are usually sent to all switch ports in the same broadcast domain. IGMP snooping lets a switch snoop on IGMP messages to determine which switch ports should receive frames for which multicast groups. Your switch supports IGMP snooping:

• IGMP snooping

improves network performance through multicast filtering, instead of flooding traffic to all ports

You could also break up your layer-2 broadcast domain with VLANs.