If it measures 23.5 inches top to bottom it won't fit the sizer. Simple as that. It's a stiff bag. A soft bag you could crunch to get it to fit and hope the GA has mercy on you but at 23.5 hard inches, no go. Did you take the measurement in depth including the handle mechanism? That's the other important dimension.
Domestic in the US airlines might be easygoing especially if you fly First and have Plat status. The question is if it will even fit in the MD-80 bin wheels first. It will probably fit sideways, though. I just used a 25x12x12 duffel on an MD-80. The length fit just so with tiny squeeze and I needed to squeeze for height a bit more. I only tried the larger 3-seat bin.
There should be enough posters here who have experience with that particular BR model on an MD-80. To get more attention you might want to consider starting a new thread for the specific inquiry. No need to let it turn into a huge discussion. Just answer the questions if UX-421 fits in either of the MD-80 bins and how? Second question, does it fit the sizer? My answer to the second question would be no from just the measures but I haven't tried it.
Till