I have no wetting control, but I have some forewarning of my messing.
I try to keep the front of the diaper over my crotch at all times, so if I start wetting, I'm just wetting the diaper like I normally would and can finish the diaper change normally (from a very used diaper into a far less used one).
I actually can't think of a time that I've started messing while changing, as I generally have enough forewarning of messing to avoid that, but given the nature and variability of my bowel continence I'm sure it will happen sooner or later.