IMO removing the breech plug is not necessary or desired.
First, the breech plug on a sidelock rifle is not intended to be removed and to try to do so will often damage the tang, the barrel (which has to be held in a vise which can damage the exterior) and the barrel/breech plug threads.
The breech plug is intended to be a VERY TIGHT fit to prevent leakage of the powder gas and repeatedly torquing the threads can lead to thread wear.
This is being currently discussed in the Post about in the Builders Bench Forum area and the problem was caused by just this sort of cleaning method.
There is always the possibility of "cross threading" when the plug is reinstalled. That of course would ruin the barrel and/or the plug.
Wear of the threads will cause the breech plug to be missaligned with the flats on the barrel. If you have a round barrel, it will cause the sights to be rotated off of their top center position. It can also create problems with the barrel pins/wedge not lining up properly.
:imo: if you are concerned with water rusting the threads of the breech plug, you should either do as I do and remove the barrel from the gun and heat the breech area over a stove burner (just enough to boil out the water)and apply a good gun oil like Sheath,
or if you can not heat the breech, use a Water Displacing fluid like WD40. Driving out water from areas like this is what it is made for.