I'm assuming you are talking about climbing onto the top of the lift truck (top of the operator protective guard). Though I'll admit I've done that once or twice, the fact is this is not an approved piece of equipment for that task and I don't see how incorporating fall protection is going to change that.

You really need to look at equipment designed specifically for the types of maintenance tasks you are doing. There are a lot of options, and many are relatively inexpensive including options to rent equipment for occasional use. The "we don't have the right equipment" excuse just doesn't fly.