Considering the accuracy advantages of sling-supported shooting, I'm surprised he doesn't want to sacrifice a few test rounds to at least see what - if any - differences there are to the point of impact.

This relates back to Lee Sporter construction - I've noticed more than a few converted military rifles making use of the rear sight protector screw (part #34, most schematics) hole for the front sling swivel. I can see that for aesthetics sake as well as practical shooting, but I also have a factory BSA Lee Speed with it's front sling swivel mount in this position.

Considering that any torque from the sling while firing from the supported position would be offset by the off-hand, this seems to be the better place to mount the front sling swivel.