Tapping the metal might work, I'd be concerned that the metal is too soft and thin to accommodate the S coil as a handle. Plus you'd have to take the handle off to sling-carry the bow. Here's an idea: Use the same holes to attach a piece of Weaver scope base, or other Picatinney type rail, then use the Parker post instead.
It has the advantage of being able to be adjusted to several positions, and completely down while carrying the bow with a sling.
I've tried carrying mine while extended, not fun. Plus it's hollow so one can hide small objects (like spare battery, or bow use permit).
One could also attach the rail to the stock itself. Or, if you insist on the S coil, drill a hole in the (inside) of the stock and use epoxy to glue in a nut or pillar to accommodate the S coil threads, pretty much like pillar bedding an action screw on a firearm.