HF SWR meter w/contact closure output?

I'm looking for a SWR meter, or a DIY circuit plan, that can provide me a contact closure when my SWR exceeds (or else while it stays under) some pre-settable threshold. Does anybody know of such a product, or an online or in-print source for such a design?

I'm trying to cobble-together a way to interface a couple of Icom HF transceivers to an LDG RT-600 remote antenna tuner, such that I can use the "tune" button on the transceivers to control the LDG tuner.