If you've got constant thrust from constant energy input, that pretty much implies perpetual motion, since energy input increases linearly with time and kinetic energy with the square of time.
If you want to say you get less thrust as velocity increases then you run into trouble with the principle of relativity.
So I'm skeptical too, though I think it's worth continuing experiments.
Sure, you can only find the mistake while repeating the experiments (perhaps, as someone else suggested, in a vacuum might be a plan).
I hadn't noticed the constant thrust problem you mention, I'm now recursing back through the equations I remember to try and work out why it costs more energy to push something that's already moving fast... wait, force applied was never anything to do with energy to start with, impulse/power were. Brain-fart resolved.
Oh right, that makes sense; constant acceleration implies constant force, and work is force*distance. As the meters tick by more rapidly, the motor performs the same work in less time, and thus the required power increases.
Which means, "a reactionless drive is like a car in space" would be a nonsensical concept without a preferred reference frame.
Wait, I get what you're saying (0.5mv^2), but isn't it relatively straightforward to get constant thrust from constant energy in other situations? Laser --> lightsail, for example.
To reply to myself, the thrust you'd get from a laser that shoots at you would in fact diminish as you accelerate away from it, as the light would redshift and thus carry less energy to you.
If you want to say you get less thrust as velocity increases then you run into trouble with the principle of relativity.
So I'm skeptical too, though I think it's worth continuing experiments.