I found a solution!

In the LaunchForm method of the windows handler, I have:

oChild = CREATEOBJECT("ChildForm")
oChild.oParent = This

That places the object reference of the Windows handler
object (This) into a property on the child form called
'oParent'

Then in the child form, I can say

ThisForm.oParent = ThisForm.oParent.Parent

This allows me to reference the calling form, because the
windows object is on the parent form.

And no parms get passed. I like it :)