In a derailleur situation, the chain is only tensioned enough to remove slack, and prevented from derailing by the derailleur cage. If they were on the power side, they'd be totally inadequate. Try them if you want; as you said, they're cheap. Make sure you have a setup to prevent the chain from falling off the pulleys, because the half-teeth alone will not be tall enough to prevent it. Also recommended: get pulleys with real bearings, because the $2.00 cheapies will only have bushings, which will be totally inadequate for the power side of the chain.

If you really begrudge spending money and have access to a deep spare parts bin, you can make idler cogs out of some old track cogs. 15-16 tooth cogs are a good compromise between efficiency and convenient size. The result is heavy but will last forever.