a good coating of aquarium safe sealert I imagine would be fine untill it wears off.
It's not the metals that are toxic, it's the oxides (rust) that form on them.
A pump running 24/7 with moving metal componants in it does not sit still long enough for it to start corroding.
You'll also find that they do not have "bare" steel screws or anything, they are 99% of the time powder coated or anodised. If not, it's a cheap pump. They are also covered in a lubricant/soft sealer, like vaseline to prevent seeping into certain areas which would do the same thing.
As for zinc, I don't know the content of it or the percentages, i've never worked with it. however, Austal Ships where i did my apprenticeship puts a zinc anode on the bottom of their boat hulls every few metres, what happens is they run an electrical current to it and start corrosion in the blocks. It attracts the elements that start the process and the blocks of zinc are eaten away before the aluminium hull starts to corrode. So instead of pulling a 100metre boat out of the water once a year to revamp the aluminium under the water line, you just send a diver down to replace the anodes. I can imagine then that this would be more lethal then the others due to it's quick decaying capability.
Also keep in mind, this won't happen straight away, but it will happen.