Hi Kate,
I am not sure I understand the exact argument you are spelling out. I think we agree that nature only use left-handed amino acids for good reasons -- if an organism produces a mixture of left-and-right handed amino acids the right-handed variants would not be able to function with the left handed ones and at best they would be useless (but expensive) to produce and most likely (as I understand it) they would gunk up the inner workings of the cell due to their interaction with the left-handed counterparts. This is true regardless if evolution created life or if God did.
My point is that if we naively imagine different cells where one cell produce 100% left-handed molecules, another produce 90% left handed and so on then the cells that produced predominantly left-handed amino acids would have a benefit over the others and, accordingly, evolution would select for them. In other words this seems like a straight-forward problem to explain with natural selection.
For this to be a real challenge for evolution, I suppose one would have to argue that machinery that only produced molecules of one particular symmetry could not have evolved for some specific reason; but I haven't seen such an argument.
If I am missing something, can you spell out exactly what your argument is?