|MadSci Network: Chemistry|
The major problem with fruit batteries is that they don't put out enough current to light an incandescent light bulb, even a tiny one. You might think about how light bulbs produce light -- and why they get hot -- to understand why they need lots of current.
You've certainly done your homework. I expect that we're looking at the inertia resulting from the fact that this site is maintained by severely overworked volunteers. I once tried to get a qualification inserted that people should use an LED rather than a light bulb, because a piece of fruit simply won't put out enough current. Have you tried connecting a whole bunch of pieces of fruit in series?
You can still buy "potato clocks" and whatnot from Edmund Scientific; they work on the same principle, but you might notice that they all run semiconductor devices with very low current requirements. So if you want results with your fruit battery, you need to use a semiconductor device like an LED -- it only lights up when connected in the correct direction, so if the LED doesn't light try reversing it! -- or something with a liquid crystal display.
You might try measuring the current across your fruit battery with a multimeter (cheap at electronics stores), then choosing a device that will run on that amount of current. Most light bulbs need several amperes, and a fruit battery just isn't capable of putting out that much current.
Try the links in the MadSci Library for more information on Chemistry.