For a small F/F timer, DC brushed motor, could reversing the motor act as a dethermaliser? Saves a servo?
I guess that could work, but remember you're still drawing power from the battery to spin the motor backwards - so if your existing set-up uses most of the battery capacity for the climb, you'd need to fit a larger heavier battery to be sure of having sufficient 'dethermaliser' time. Also, unless you have radio control assist, you'd need a way of detecting that the plane had landed (or preferably just before landing) to stop the propeller spinning - otherwise you might damage the propeller/motor or drain the battery.
The lightest solution is still likely a pop-up tail activated by a rubber band and using one of those systems that has an electronic timer: when the timer elapses, a short piece of wire is briefly heated to melt through a tiny retaining rubber band: the so-called 'fuse'.
This is using my pulser boards in PWM mode driving the coils from the flight battery (2-cell) rather than the ESC BEC. I had a couple of successful flights using the BEC but the combination of elevator and rudder coils were drawing about an amp from the BEC - which was its maximum rating.
Now the coils are driven PWM from the battery, there is no load on the BEC. However you can't push the coils much above an amp, or they start to get hot. I run the PWM at about 50% to stay below the 'heat limit'. It could still use more actuator power - I think rewinding the coils with more turns of thinner wire and/or using bigger magnets is the way to go next. It's quite flyable as it is, but I need more control power if I want to indulge in non-risky aerobatic manouvers!