Would wearing a mouthguard when boxing be enough to protect my veneers?

Taking a shot directly to the mouth, even with a mouth guard in can cause damage. Your whole jaw could break even with a mouth guard in place. That is the risk you take when boxing. It might prevent minor chips from the jaw being nudged, but any blunt force trauma to the jaw could cause breakage to the veneers, to teeth, and jaw even with a nightguard in place. You wear it to hope it works, but no guarantees. Sorry.