If Exolent's method doesn't work, I think your best bet is to modify a bunch of pev data on it, piece by piece, and see if you can get any change, or even going as far as to messing with pdata. I know I've gotten some strange results on entities by modifying this stuff.
In fact, a func_door will turn invisible (but will still be solid and make noise) when you set pev_iuser4 to anything other than zero. Try that one first.