Fixed it. ThinScript.dll was the wrong guess. I found a reference to registering Plan.wsc in an unrelated Customer Service post and that did the trick. Along with PlanChecker.js, Plan.wsc, which looks like it contains the external compiler, seems to be the only other file necessary to run the Plan Checker standalone on another Windows machine with ASCOM installed.

When running 'regsvr32 Plan.wsc' the registration reports an entry in \Windows\System32\scrobj.dll, so it would appear that was affected by the Win10 update.

Best to all,