Measuring torque at the crank or the wheel is how all commercial power meters on a bike already work. The most accurate way to develop a load cell to do this uses strain gauges which is fully detailed on my site already mentioned in this post. (keithhack.blogspot.ca). There is a video tutorial on how to install them. Torque is just perpendicular force x distance from the axis, power is just Torque * omega. (omega is rotational velocity in Rad/s).

Several other places online have mentioned trying to measure chain tension with a spring loaded derailleur. The short answer is that it isn't accurate, you've got too many things interfering causing it to move around - plus, if this isn't on a fixed gear bike then you have to deal with the chain moving. If you do some digging you'll find that Polar originally developed a device that senses chain natural vibrational frequency which is related to tension. It was the most accurate of the chain methods - and a market failure due to lack of accuracy. +/- 5-10%.

There was talk about measuring against something "resistive". I can only assume you mean the opposing forces on a bike, namely acceleration and aerodynamic drag, which accounts for 90% of a cyclists power output at a constant speed. Remember sum of forces = mass * acceleration, and that there are many forces. If you have enough information on acceleration, air speed, air density, a guess at your Cd coefficient, and drive train loss you can back out the power input... however it's been also regarded as inaccurate. +/-5% at best, more like +/-15% in worse case scenario if your Cd is way off. See ibike. It's best used in conjunction with a real power meter which means it gives you your drag coefficient in real time.