Mathematically, you get your hands on a true compact ( for example a case 1825b @ 3000lbs), and get a 1000lbs plus the mast itself @ 12', my UN-educated guess, you will have a real liability on your hands. Full size machine, wont be rock stable, but would handle the load, and stay pretty stable (assuming you dont accidentally bump the controls from side to side or back to front, which is really easy to do, and could have catastrophic results with your load at 12'). If you make a miss-step at the controls of a compact, with that load, at that height, and it could probably easily all come "undone". This is my guess, Im not an engineer, I dont play one on TV, and I did not stay at a Holiday Inn Express last night. I would seek a professional opinion on this one. Your question, is really geared for an engineer. I will say this, I would not do it. Not unless I was ready to sacrifice the machine, the mast, and maybe my life, and whatever your going to lift. Thats based on a compact, like a case 1825b. Now a full size machine, as long as your good at the controls, its very doable, safe-no, osha compliant, surely not. But as stated my neighbor has been doing it for years (his machine is 6000lbs), but at half your load, and twice the machine. If it was me, Id buy and off road forklift, or a pneumatic tired forklift (not sure what you conditions are and what you are trying to do). But I understand you want he utility of the loader for other work. You can always try, and if it does not work out, you will know that the load was too heavy. BTW, I do have a pneumatic tired hyster forklift, and I have pallet forks for my loader. Turns out a loader does not always function as a forklift, even if it has forks on it.