Good Pics. From the looks of it, the block that the hand lever attaches to(the portion that tilts up and down) might be able to be limited in it's travel by drilling and tapping a hole in the top of the outer frame(part that rocks left and right, near the leading edge) and installing a setscrew. When screwed in, this would limit the amount the block tilts forward and stop the lever from pulling the plunger out as far thus limiting it's lowering speed. Not having the component in front of me though, I am not sure how much this would weaken the structure. You could experiment to see how this would work using a screwdriver blade or chisel head held in the gap above the up/down block to see how an obstruction there limits the lever travel/lowering speed. Another option that might be easier would be to attach a plate to the existing mount using the two mounting bolts that hold the valve to it's mount. The lever would run into this plate and limit how far the stick moves forward. By changing the position/shape of this plate, you could possibly limit the lowering speed.
Does this valve have "Float" capability? Limiting the lowering travel would prevent it from going into floating mode.
I grew up using 2 lever spools and have only used a joystick on a few occasions. If you are not into some light machine work, you could probably get a local machine shop to modify this part for you pretty quickly. A blocking plate could be made pretty easilly with a drill, grinder and jigsaw.
Replacing the valve with another might be a better option. Northern tool has a 2 spool loader valve with float on sale for $159 right now. My current loader valve dosn't have float and I really miss having it for fine gradeing, so I have been looking at this myself. My personal preference would be to install a 3 spool valve($234 at northern) as mentioned above for future expansion to control a grappel or front mounted trencher bucket. I have never owned a Chinese loader so don't know if there would be a confict with the hydraulic hose threads vis the standard threaded fittings found on a domestic valve.