A fellow lifter at my gym is actively pursuing an 800# deadlift and his sole limiting factor is grip.
In particular, I have noticed him performing two motions to improve his grip.
The first motion is double-overhand deadlifts.
The second motion is an special grip exercise. To perform this exercise, he graps a thick handle, which is connected to a metal plate with a tube (structually similar to a sled) and on which plates are loaded, and holds the weight up until the handle slips out of his hand.
As you do not have a similar apparatus, I am not sure how you would employ the second exercise, but I felt I would share what I know.
Kroncke