I had built an inline aeration contraption out of PVC, using a bit of tubing and silicone caulk to attach my barbed aeration stone to the gadget. Drawbacks: couldn't bring the PVC above ~150, so had to permanently store it in sanitizing solution, and the caulking just didn't stick well enough, and the tubing popped off with the aeration stone. But for the $3 in PVC it cost me, good learning experience
I just rebuilt the contraption out of copper, should be far more solid now, and you can remove the aeration 'insert' for the lack of a better word, whereas the PVC one I made was more enclosed. Just finished it yesterday, and more of an initial prototype. Brewing today, so we'll see how it works. The only not as solid as it could be part was joining the barbed aeration stone to the 3" nipple. I tried using copper solder, no luck, it won't stick to the stainless, and didn't want to try brazing the stainless onto the brass as I don't know if that would work nor whether the stone itself would handle that kind of heat, so I used a bit of JB weld and shoved the barb inside the nipple. We'll see if it holds.
Q1: But I do have a couple questions for you guys: where the crap do you find the aeration stones with threads? No matter where I look or how much searching I do, I can only find the ones with barbs. However, if I were to make one again, I'm going to order a stainless steel 1/8" x 3" nipple so that I could shove the barb end inside it, and weld it together, the SS nipple was probably only a buck more than the brass one.
Q2: And since I enjoy making this type of stuff and providing custom equipment to my fellow home brewers, how many of you would like to have an inline aeration gadget? Would $60 + cost of aeration stone (as this is the big unknown) be reasonable? If interest is high enough, I'll make another one.