I think it's a good idea and much safer than a lot of options. But there are two important changes I would make:
1. If you are to be standing on this, the low sides and bucket back are a huge hazard. They provide an easy way if anything happens for you to flip over "head first" towards the ground. I would raise those up to railing height, plus they're really easy to trip over.
2. I would use plywood on the 2x4 sides to reinforce them. It doesn't see like racking would be a big deal with the ratchet straps, but it will stiffen them up a ton and prevent any issues from developing.
It wouldn't do much for the bucket roll cylinders, but locking the loader lift cylinders is pretty simple. Just get a piece of angle iron, maybe like 1 1/2 x 1 1/2 and cut it to whatever length it needs to be to hold the loader at the chosen height. Just lay it on the cylinder rod and lower the loader till it is held there by the cylinder. This assumes of course that you're dealing with a set height for the loader, or perhaps cut a couple different lengths of angle and use whatever is needed.
I was thinking that if you want to create some simple cheap locks to inhibit a disaster if hydraulic failure occurs. I would find some schedule 80 PVC that could be cut to 'snap' around the 'cylinder ram' at the desired height. measure for length, cut vertically to perfectly semi-encapsulate the rams- and voila' !!
Note: this is just thinking-out-loud; I have no idea if this is feasible or not. Thought's? I do- think that they would support the combined weight of the loader and it's human cargo though.
Get a removable lift platform for your pickup truck where you can work SAFELY and efficiently.
VIDEO: https://www.youtube.com/watch?v=5sWWtZIPEqM
Lipstick on a pig does not change the pig.