Not sure if Brocaar added the refactored mesh heartbeat that you can adjust the payload to, but if not this is the general steps that would be necessary.
It would be messy, especially to have them uplink through Chirpstack and appear in the UI. Although I’m sure it is possible, for example there is a LoRaWAN simulator out there to perform stress tests on your network, it creates “fake” Lorawan uplinks from different gateways, I believe it works by sending UDP packets to your gateway bridge that mimic real gateway uplinks.
The extra layer of complexity here is that you want to use the gateway mesh, which typically proxies traffic directly to/from…