With exception to standard good mornings (bar on back), and grip work, as that isn't a problem, what is the best assitance work for a deadlift? I have access to a barbell, adjustable dumbells, and a sandbag.
Pin-pull (partial deadlift), glute-ham raise, cable pull-throughs, reverse hypers and seated good mornings.
I think pulling off of a box or any raised surface will really help your dead, especially if you are weak off the bottom. Also snatch grip deads helped me a ton but I needed straps to hold onto the heavier weights.
Stiff leg deadlifts off a hundred pound plate or a 4 inch platform. Keep your head up through the entire lift and keep the bar close to your shins. These are by far the best assistance move for your DL. Especially if you pull conventional. As a matter of fact this is the only exercise I will probably do for my pulls until I peak for my next pulling contest.
What is your sticking point in the deadlift?
Of course the Reverse Hyper, 45? hyper, and Glute-ham-gastroc raise are excellent, but they require machines.
Since you only have barbells and DBs, I would recommend stiff-leg deads with bar or DBs, Dumbell swings/snatches (swings are a great replacement for pull-throughs according to Louie Simmons).
Also Romanian deadlifts, Zercher squats, and maybe barbell hack squats (many Fins do this and they are notoriously good pullers). If you want an alternative to the GHG raise, weight your feet down and do "natural" glute-ham raises.