Wouldn't that require a constant filter, to ensure that L-processes never use R-amino compounds? Seems almost like a Maxwell's Demon would be required in every cell...
They just can't use those compounds. Many medications have L and R forms of which only one is active. Right-methamphetamine is, well, methamphetamine, but left-methamphetamine is a nasal decongestant. Which is to say that the unwanted variation of a compound can be harmless.
Just about every molecule in every cell is already a maxwell's demon. They only fit together in specific ways.