Mark, have you ever tried to use helium on a tig rig without HF box to weld aluminum. I have and it will work. You have to work it to get it to tie in since you dont have the vibration of the puddle like the Hifreq rigs do, and it has to be super clean (black sooty contamination will not weld)
You said it could be welded with AC buzz box, not DC.
A DC power source can weld aluminum with electrode positive, but you need large tungstens for even the lowest amperage and pure helium. But I have never found anywhere, that an AC box without HF can do a suitable job welding aluminum. With the soot issue you are talking about, and the tungsten contamination from touch/scratch starting it, you would NOT have a quality weld, and that is the point.
You can weld cast iron with about any process you want, but the weld strength and quality will vary widely. Its the same thing with Aluminum. What you can do and should do are entirely different.
I have calls frequently from customers who are frustrated because someone on the web told them they could weld aluminum with pure DC, and they bought a DC TIG from us and they can't do it. They say their electrodes are burning up. (Probably hear it from the same guys that told them that 75/25 gas should be used for tig as well). I ask them what size electrodes they are using and amps...They tell me something like 1/16 and running about 40 amps. No wonder, even to do that range, you'll need a 1/4" inch electrode to withstand the heat. It could be done with DC - as well, but with the problem of instant oxidation of aluminum, the weld will never be quality.
Yes, it can be done, but its not for some beginner, or someone without the knowledge of whats going on. And there is a practical limit to what can be done. But they think that because someone said it, its a perfectly acceptable and easy way of doing it.
So, from my standpoint, while a technical answer is yes, you can do it, the practical answer, is no, you shouldn't/can't, because of your inexperience and lack of knowledge.