>>>If I'm not full of hot air, I believe that some of the Airbus family uses the radio altimeter to inhibit/allow TR use close to the ground. ??
No your not full of hot air. Most modern airliners utilize the radio altimeter system as a redundancy to wheel spin-up to deploy the thrust reversers.
Here's why. Most later model aircraft utilize main wheel spin-up as a primary "on ground" signal allowing the thrust reverser to be deployed. But certain scenarios could potentially nullify this from happening. For instance, severe hydroplaning can prevent the wheels from spinning up potentially preventing the t/r's from deploying.
The reverse thrust system use's the radio altimeter as an alternate method of determining "ground" mode to deal with the contingency of ice or water on the runway preventing wheel spin-up.
The radio altimeter "trip" height is very low, 7 feet on the MD11 for instance. This height roughly commensurates with actual radio height with the main gear struts extended touching the ground.
The radio altimeter is also used by the auto-spoiler system for the same reason.
You're only as good as your last departure.