It's been a couple of years since I completed this build so I thought I'd provide an update.
I finally added those float switches to the HLT I was thinking about.
The bottom one is inline with the SSR control circuit and is closed so long as the water level is above the element. When the water level drops below the element level, the switch opens breaking the circuit and the SSR goes to it's normally open position. Therefore no power to the element.
The top switch connects to the solenoid valve on the water line. That valve is normally closed. It opens when power is applied to it. The float switch is inline with the toggle switch installed with the pilot light on the front of the control panel. This float switch is situated such that when the water level is to the top of the HLT, the switch opens, cutting the power to the solenoid, which then returns to it's normally closed state (so long as the water pressure isn't too much).
Both connections are routed through the back of the control panel with barrel connectors so I can pull the HLT off the stand for maintenance.
To date, I have not dry fired my HLT element nor overflowed the HLT since installing those switches. That's a win-win!
I don't think there would be much else I would change, except make the water filter hanger more sturdy and use a stronger solenoid valve or one that is NO instead of NC.