Finally got around to this, but had a couple of issues. First of all, the slash (/) has to be in this direction or you get a syntax error. This is what I ended up putting in the 'Screen Load Script':
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-- Screen Load Script
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pageId = 0
screenId = 0
testcount = 0
machState = 0
machStateOld = -1
machEnabled = 0
machWasEnabled = 0
inst = mc.mcGetInstance()
mc.mcCntlLoadGcodeFile(inst, "C:/Mach4Hobby/GcodeFiles/O0001.ngc")
mc.mcToolPathGenerate(inst)
This worked to load the default toolpath, but now I don't get the normal startup screen. This is what I get (see attachment). What did I do wrong?