There are no explicit APIs at this moment to indicate that data is being sent over the air. The closest approximation that you could leverage would be to hook into one of the bus functions like - wwd_bus_sdio_transfer - which will indicate valid packets being transferred to and from the WiFi MAC layer.
This is a hacky way to accomplish what you want however.
Recommend - that you instead hook into the wwd_thread_receive_one_packet and wwd_thread_send_one_packet to mimic the functionality
Keep in mind that all of these are low-level functions and may potentially change between SDK revisions