Known drug house is obscured by acres of brush so that it is not visible from the road. Is it permissible for the officers to walk through the unfenced (yet owned) brush to watch the activity at the house?

Nope. Not if it's private property and they don't have any other legal reason to be there. They've got to be standing someplace they're legally allowed to be standing for plain sight doctrine to come into play.

Is the brush owned by the same people as the drug house? Then you're SOL. If it's owned by someone else, ask their permission to walk through it. Then plain view will come into play, because they'll be somewhere they're legally allowed to be.