TiGs Axel showed me how you could do this with a space of only 1.5" from top of the tank to the water line. Instead of clamping through the usual clamp notch in the rail you clamp further down through the cogs that the sensor clip locks into. This allows you to move the sensor exactly up to the clamp. The safety sensor can be closer to the optic sensor than 1", it could be as close as a half inch but the problem here is the displacement from working in the aquarium can trip the alarm which is really only an annoyance. Given this possibility the only issue is snails crawling on the sensors.
What we were working on and I can say with 90% certainty will be built is a mini Comline filter system for tanks up to 50gallons. It will work with our 3110/2 skimmer and be about the same size and basically function as a filter for use with chemical or mechanical media and a sensor holder. This layout makes the sensors more accurate (like the current Osmomat) and it makes the sensors safe from interference from animals and debris. It will likely not be ready until next summer but should be a great set for mini reefs and will be available as components individually or likely as a Mini Reef Pack filter. Like the current osmomat, it would be possible to be 1" from the top.