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'.