What I'm asking is whether they can be mounted horizontally close to the substrate and thus hidden by plants and hardscape. (Seems to me there would be less of a chance of air exposure that way anyhow.)
I don't see why not. I am sure Lowe can give you a definite answer. I use mine vertically and love it. It is the best heater I have ever purchased.
Edit: The only thing you need to look out for is where you put the sensor end of it. If your water level gets below it, the sensor will start reading the air temp and run the heater until the air temp changes.