I finally worked it out:
JXA Script to Get/Set Local & Instance Variables
var app = Application.currentApplication()
app.includeStandardAdditions = true
var kmInst = app.systemAttribute("KMINSTANCE");
var kmeApp = Application("Keyboard Maestro Engine")
var myLocalVar = kmeApp.getvariable("Local__MyVar", {instance: kmInst});
kmeApp.setvariable("Local__FromJXA", {instance: kmInst, to: "Set in JXA Script"})
Here's my simple test/demo macro:
##Macro Library Get Local Variables in JXA Script
VER: 1.0 2017-10-11
####DOWNLOAD:
Get Local Variables in JXA Script.kmmacros (3.1 KB)
Note: This Macro was uploaded in a DISABLED state. You must enable before it can be triggered.