By not putting their small number of 787s on their non-existent hub-hub routes.

The problem with your idea isn’t the two-class planes (although, as WineCountryUA points out, they may suffer on comparative booking sites. The problem is on the three-class planes. According to your plan, I’d expect to be able to buy “J” on IAH-EWR on the two widebody flights a day. Why is that a problem? Service recovery. Now, you get MX or WX and you’re trapped — you’ve gotten people to pay extra for lie-flat seats, so now you can’t swap in recliners when necessary without paying a big chunk of change in downgrades.

Anyway, even if it wouldn’t create operational headaches, I see absolutely no value to UA in doing this. Most people don’t have any idea what “J” means — they know “business class” or “first class.” And even “business class” has a varying meaning — what do you do with European business class, which is sub-PE? You’re trying to solve a problem that doesn’t exist. UA has chosen not to monetize the PE seats on non-PE routes. If they chose, they could change that policy by adding in O/A/R buckets.