You have finally stepped into my realm of expertise...I have built a few minitrucks with airbag suspensions and i know the answers. To start, ABSOLUTELY do not using anything from Airbagit...they have the worst reputation in the business. Ok, now past that.
First of all, you will need to find a way to convert the pressure sensor signal over to a signal the range rover computer can accept. This unto itself can be complicated. Secondly, its not a bad idea, however, the weight of anything will increase the pressure. Therefore, if your system is set to maintain the same pressure across the board, any weight on a corner will make that corner lower, not only because you are weighing on it, but the computer will drop the pressure to maintain, creating a never-ending, painful cycle. The height sensors work because different pressures can be used to create certain heights. Then, lets say you set up each corner for a different value, but if you put more gas than you planned on, or more passengers, or a different driver, or tow a trailer, you are up the creek...
Point being, pressure sensors would be much more problem than they are worth, considering how you would have to plumb them into the system and calibrate (what we are trying to avoid in the first place) the WHOLE system, not just heights. It seems great, and you have the basics within the Range Rover system, but it is just not even close to feasible.
Realistically, to do it right, you need to remove the Range Rover system or bypass it and put in your own system, from top to bottom, including valves, ride control, sensors, gauges, etc., possibly using the factory lines and bags. An interesting thought would be to have a manual control option to lift when wanted or lower it for that boulevard crawl.