Try using a small bungee cord. The elasticity will keep the pressure on the valve while working and you can always move the lever back with the bungee cord still attached. Won't fix your situation permanently, but will allow a work around.
The pressure out is forward which puts a bungee cord in the way of where your feet go. I've been looking at it and have thought about tack welding a small hinge to the decking where the handle extends to that way I could move the handle forward and flip the hinge over to lock it. So far thats all I've came up with.
I relooked at it and a hinge wont work because of the deck angle to the arm a hinge wont clear. I'm thinking maybe a slide lock might work, should have enough clearance. I just wasnt coming up with anything a you got me started thinking (sometimes that can be dangerous) lol – thank you.
I went to Rural King and picked up a slide lock. It works great. Push the lever forward and slide the barrel over. The nuts and lock washers were a real bear on the back side of the decking. I had to get the wife with smaller hands to do two of them. It's done and works good!