With a weight mismatch, I don't think redirecting an upper belay through the anchor is a great idea. Some kind of direct belay off the anchor seems preferable to me.

But if you use a guide plate and accidentally thread the device backwards, so that the braking strand is not pinched by the load strand, you may get very little resistance to a fall load.

Even with so-called autolocking devices, belay gloves are a good idea.