this might sound stupid but i've had similar issues and one of these 2 options may work ??
try to tighten the plug .... it may break the seal and allow you to unscrew it ...
or ..
if you can get a hammer in there with a bit of swing, hit the plug 'into' the gear box .... can have the same effect as tightening it by shocking the seal.

might be worth a try but just thinking of it, the plugs are tapered aren't they ?? give it a go anyway ... doesn't sound like you've got anything to loose