No doubt, this is a tough situation.
Of course you're not going to get management to sign a piece of paper declaring the equipment as "safe". Duh! Safe is a relative term and no piece of equipment is absolutely safe.

As a trainer, you need to train the operators on the potential hazards of the specific equipment and environment in which they work. Most motorized pallet trucks have no operator safety cage at all, yet in the appropriate environment with the appropriate training they can be reasonably safe. That being said, there are environments where these same trucks do not offer a reasonable safety level and therefore should not be used.

I think you need to look at the vehicles and your specific application and make some type of determination as to whether or not they are reasonably safe with proper training. I realize that "reasonably safe" is subjective. If in the end, you feel the use of these trucks create too high a risk, you shouldn't use them, period. Whether or not someone else signed a piece of paper saying they are safe doesn't change anything. Be aware that refusing to use them is a pretty strong stance and may result in some very bad things happening to your career within that organization. Life isn't always fair.

I know it's a little late to say this, but didn't you folks at least try a demo vehicle before purchasing a bunch of these?