I think the answer is "no"... at least I haven't been able to figure out how to do that. For most the levels where the code is re-used over time, I just grade the last level that uses that code - usually it builds up over time (I am thinking of the clicker game or colorSleuth , for example). I agree that for lesson 12, the tasks vary more over time and totally get the desire to be able to see the in between versions.
There have been a few times I have suspected academic dishonesty and wanted to see the version history for a student project. For those I just have a student pull out their computers and I inspect the history on their machine, but that is time intensive in class to do for a whole group...
I think this is still good feedback for code.org to hear though, can you "submit a bug" ticket just to more formally log this feedback into the system. I can submit feedback too, I am guessing it was designed like this for a reason - submitting a bug might also get you a more detailed answer on why it is setup its current way.
Thanks again for raising this!