Easy access to the inner 6mm bolt, at least on the driver's side. Haven't done the passenger side yet.
Tools:
Stub of 6mm allen wrench with no bend in it (got mine from ************** in their little motor mount tool kit)
1/4" 1/4" drive socket
1/4" wobble extension
Masking tape.

Jack up the engine.
Tape the stub of the 6mm allen into the 1/4" socket so it won't drop out as you lower it down.
Put the 1/4" socket on your wobble extension. (may work with a standard extension--I didn't try. I went with the wobble) Make the extension long enough so you can get the 1/4' ratchet on the extension way up near the top of the engine so it's easy to work with
Reach down on front side of engine mount arm close to the IP with the socket/wobble in right hand while using left hand to direct the 6mm allen into the inner bolt. (clean out the head of the bolt in advance-I use brake cleaner aerosol with a 2' extension nozzle)
Unscrew the inner bolt
Voila.

I should add that you need all this in 1/4" because a 3/8" drive 6mm allen socket is too fat. It will hit the motor mount arm and not be capable of going in to the bolt head.

All this on 84 SD

Finished the passenger side mount. I can confirm that once the air filter bracket is removed you can access the inner bolt of the mount using the tools above by feeding the long extension down between the manifold and the turbo.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13