Someone kind of had the same idea as an actual product, you can find stuff like insertable absorbent pads you can put into a toddler's undies to turn it into a trainer pant, like Dry Like Me.My mother used to put a Pamper in my underwear without taping it in place. That was in the early 80s when I was around five to five-and-a-half years old. That was before pull-ups existed by a number of years.
This essentially functioned like a pull-up (at least most of the time). It wasn’t perfect, though. I seem to remember being found out in preschool when I was wearing underwear with elastics that were too loose in the leg area and the small to medium size Pamper fell out. Fortunately it was dry at the time. I’m guessing that avoiding this problem is why the Goodnites inserts that I used to see for sale in some local drug stores had some type of adhesive on the back.