Hi Jon, I've used hydraulics in my featherweights for a while now, so they can be made to work. As Colin says they can be slow, but you have so much power there for a lifter, you could use a linkage to speed it up.
The biggest problem with those sorts of power packs as you have identifiec is the leak potential from the breather when it is inverted. To get around that I have used a bladder for the tank then given it some sort of secondary protection to stop it being punctured. However you need to be careful that when the amount of fluid in the tank is at it's greatest that the tank does not become pressurised so that you do not fall foul of the no accumulator rule. You also need to ensure there is no air in the tank as you do not want to end up in a situation where your pump is trying to suck air in and it stops working as a result.