"Mxsmanic" wrote in message

All well and good, but that doesn't resolve the problem I've raised,...

The problem here is that the presumed weight for carry-on luggage
right now is some non-zero number x, when in fact the real carry-on
luggage weight is zero.

I think you may be overthinking this. I would think the general presumption
to be that the bulk of former carry-on luggage is now being checked, so the
total weight remains about the same, just shifted to a cargo hold.