The rear freewheel won't come loose, I don't think. It is the derailleur that will unwind. Currently the derailleur is springloaded to keep chain tension while allowing for varying chain lengths for shifting gears. Using it to drive the front chainring as you suggest will unwind the derailleur causing the top of the chain to loosen and fall off. Try this on your bike. Hold your front freewheel/chainring from spinning, then roll your bike forward. You will see your derailleur unwind. It would continue to do so until it ran out of slack and then it would actually turn the engine over. In the meantime the top of your chain now has all the slack your derailleur was taking up and will probably fall off.