On my 1993 400E, I was able to lock the crank by opening the access plate on the bottom of the torque converter (2 small bolts), then turn the crank clockwise (facing the engine from the front) until you see one of the three torque converter bolts. Pull one of the bolts out and replace it with a longer (3") hardened bolt and a 2 1/2" hex-shaped sleeve. You can get both from Ace Hardware (I think it is an 8mm, but get the hardest they have).
Once you tighten the bolt, the outer sleeve will keep the torque converter from turning as you unbolt the crank bolt (counterclockwise when facing the front of the engine), because the sleeve will hit the transmission housing. You may need to add some extra nuts along with the sleeve in order to compensate for the bolt's length. Here are some pictures in this string that may help, although your setup will obviously be different, this is how I locked the crank on my 400 without the expensive tool.
http://www.peachparts.com/shopforum/showthread.php?t=190442&highlight=560+water+pump
My crank bolt was so tight that I actually had to use a floor jack under a breaker bar to loosen it and it nearly lifted the engine off the mounts before it came loose. If you're going to disturb the crank bolt, you may want to think about replacing the front seal while you're at it. The seal is right behind the balancer and coupling. On my car, there is a special tool to pull the coupling that goes around the crank and another special tool to install the front seal. I think you could probably get away with the generic pullers and a big socket to install the seal, though.
However, if you are trying to lock it up for reasons other than loosening the front crank bolt (e.g., to keep the engine from turning when you pressurize the cylinders to replace valve seals), then I doubt the above method would work.