I have the same interest. I've been experimenting with my 5-stage RO system. I put a pressure gauge after the pre-filters and just before the membrane, but it never really shows any drop in pressure. Not enough flow to show the effect of a really dirty filter. The carbon filters are actually used up long before it would show as a pressure drop.
Another way to do this would have been a FLOWMETER. This way one can measure the amount of water that has passed. Filter replacement would be triggered based on the capacity the filter manufacturer states.
For whole house filters and high flow rates, the pressure gauge would seem like a good idea. A differential type pressure switch could automate this?
In the end for the RO system, I got a chlorine detection kit that turns the water pink if chlorine is present. At six months, I tested the water after the first, then second carbon stage and found that chlorine was beginning to appear on the output of the first carbon filter. So I now change the first carbon every 6 months, and both carbons and the sediment filter every year.
On ebay, you can also get in-line TDS meters relatively cheap. But if one waits to see the effects of a failed membrane, it is too late!
Good Luck!