Jeff please feel free to give me the old "I told you so" haha
No way. I really wanted it to work out.
If you are going to make the aluminum tube one continuous piece, and you don't intend to get nuts with really heavy bands, then yes, i think that it will probably be strong enough. And you have exactly the right idea by "counter drilling" a little to leave the stainless at the full diameter, and then starting the threads a little deeper.
For what it's worth, and at the risk of letting the inner nerd out, the stiffness of a round tube is proportional to: OD (to the fourth power) - ID (to the fourth power). So if you wanted to compare the stiffness of 5/8" solid rod to 1/2" solid rod (of the same material), for example, then the ratio would be:
(.625 x .625 x .625 x .625) / (.5 x .5 x .5 x .5) which equals 2.44 . So the 5/8" rod is 2.44 times as stiff as the 1/2" rod.
The reason I say this is to emphasize that a small change in diameter can pay big dividends in stiffness. A 5/8" tube can have a thinner wall and be the same weight as a 1/2" tube, but the 5/8" will still be much stiffer.