Sorry, I thought you can put 1-day Travelcards on prepay. This is what
the TFL website says:-

"During any 24-hour period from 0430 to 0430 the following day, you
will never pay more than 50p less than the equivalent Day Travelcard
price for all your Oyster single journeys in Zones 1-6 or we will
refund the difference. "

I read this as meaning that if I make dozens of journeys within, say
Zones 1 and 2 in 1 day, then my Oystercard would be charged the
equivalent of a 1-day Travelcard for Zones 1 & 2, less 50 pence. Is
that not the same, in all but name, as buying a 1-day Travelcard on
Prepay?

Depends if you think the capped Oyster fare that is valid on only some
National Rail lines is the same, in all but name, as a 1-day travelcard.