Hi Eightbit,
I would assume that it is setting a modbus register with some value on a fixed interval. This would mean the watchdog client they supply has the fields to supply the connection (meaning the watchdog is the modbus master). If you could figure out what register / value is being set (and that is indeed what is happening), you could have Mango perform that action, probably through the Scripting data source calling .set() on the watchdog point, but there are options as well.
I would expect it to be in the device's manual.