Can multiprocessing with arcpy be run in a script tool?

I have been tinkering around with the multiprocessing module to resolve some processing of data in ArcMap 10.3.

I have created a script which works fine if I run it in IDLE. I see all my cores max out in Task Manager and the code completes without error.

Now if I wire this script up as a Script Tool in ArcToolbox it throws a weird error

Could not find file: from multiprocessing.forking import main; main().mxd

Now reading various threads I regularly see that the script has to be “run out of process”. So I unticked the check box “run python in process” in the script properties dialog and was expecting the code to run but it does not, I get a 000714 error and I’ve done nothing to the code.

So my question is simply can one create a script that uses the multiprocessing module and run it from ArcToolbox? From my limited playing around, it seems I cannot and its a technique that can only be run from IDLE?