Chain elongation is measured over a certain length of chain, but what you're really looking for is the % of elongation. Of course if you double the length that you're measuring, and if the % remains constant, then the total elongation you measure is doubled too.
I'll try the experiment, DD. Although it may take a bit to set up, maybe using a gate to the paddock. I know that I can start a road bike in a arirly high gear, i.e. 52/14, and with some effort and hard pushing get up to speed. In fact, that's how I've broken chains. Doing the same on my recumbent takes much longer, implying that I'm NOT pushing as hard.