Hmmmm, do you have a can of spray lacquer? you could coat it with that. It will be invisible, but leave a coating on the stem site to protect it.
That stuff is nasty and may also kill any molds and bacteria already on the site. That is the problem with knocking a stem off a pumpkin, it has a tendency to develop mold and rot there.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-