You have to build something on top of Netwatch tool. You can not use it directly because there is no way to set the Netwatch tool to use a specific outgoing interface.
I have asked for these options for Netwatch tool, MT is considering it (http://forum.mikrotik.com/t/little-changes/47848/1):
we will consider about src-address for netwatch tool. In the mean time use the scripting and ping to check something this specific.