Just shooting my mouth off again, in a previous life as a Massey Ferguson mechanic
we would change the plugs out by hitting one side with a hammer and refitting with socket and hammer with a hard setting goo. Anything none hard setting acts as a lubricant in structural joints and can cause the plugs to push out under pressure. In a later life as a Engineer
for MF I learned that the real purpose of the holes is to support the 'core' of the casting ( the core
creates the holes in the block for the cooling
passages and is a resin impregnated sand that is vibrated out once the casting solidifies). But they also work well as safety
valves if it does freeze. No real point to this post just adding / agreeing with what's going on... I like the rubber bung idea as an emergency
fix I'll look out for some.