Hi there
By 10.12 you mean OSX Sierra right? That doesn't sound great! I know in 10.11 I'm able to work around the unidentified developer.
Since I'm assuming you're using a technical preview or something, are you able to use Mnemosyne on a different computer/partition until Sierra is officially released? Then I'll be able to update my computer, try it out, and see if I can work around the problem.
If you know how to register as an identified developer, that would be great, and I could go through that process if it helps. Let me know if you have any more details on that!