Well... I can tell you what the US Navy
does on a Carrier if they have to break the connection between the hull
penetration and the valve.
The put a "cofferdam" over the penetration. A temporary box welded to the side of the ship to keep the water
away from the hole.
The plunger idea is very similar... but missing 2 essential components:
to the hull
Apply some sealant
. Non-hardening so you can get it off.
A way to ensure it doesn't slip.
The sealant will help... but run a line ALL THE WAY around the hull and apply pressure on the plunger to make it secure.
You aren't dealing with 30 to 40 ft underwater and a 4 ft diameter hole in the hull... So you don't need as robust of a cofferdam as we used on the carrier's main seawater inlet.
If your "exterior plug" is able to handle the pressure and stay in place it should be fine if you are not moving the boat. Especially since you are planning something that should only take a couple of minutes.
And for safety
: Calculate the max flooding rate through the hole and have a sump pump (if you have shore power
... maybe $50 or $100 to purchase) that can handle 2X that rate, in addition to your float operated bilge pump
Then we get to one of my pet peeves...
Boat makers don't put in adequate bilge
pumps. Your AUTOMATIC bilge
pumps should be able to pump out at a rate at least equal to the worst case single
through hull pipe shear. Better would be if the bilge pump
is rated for double that. (after the loss of rate due to the lift
to get the water
up to the pump's through hull)