Well, firstly most of the concentrates sold are complete bunk. They are supposed to contain live bacteria, but if the bacteria are alive, and need oxygen to work, where can they get that sitting in a sealed container on a shelf? This is why Bio-Spira must be refrigerated the entire time before use, if it gets too warm, the bacteria come out of hibernation and will die because they won't have the resources they require. If that weren't enough, pretty much every single prodcut sold in the U.S. that is not Bio-Spira actually don't have the correct species of bacteria in them. So, even if the bacteria were alive in most of the bottles (which is unlikely), it isn't the right species anyway!
Also, the theory that "the more waste there is in a tank the more bacteria will grow" is bunk, too. During the cycling phase, the bacteria are reproducing and growing as fast as they can. They have more than enough food. The size of the bacterial growth is actually dependent upon the rate of production of ammonia. The bacterial colony will grow until the rate of consumption by the bacteria is exactly equal to the rate of production by the fish. The bacteria doesn't care is there is 1 ppm, 2 ppm, 3 ppm, 4 ppm, or even 0 ppm at any instant. All that is important is what the rate of ammonia production is. This is why water changes -- even very, very large ones -- are perfectly fine to do during cycling. Yes, the amount of ammonia in the tank is reduced, but the rate is determined by the fish which is exactly the same before and after a water change (assuming you didn't suck up a fish!).
Lastly, you'd have to figure a way to grow only specifically the species of bacteria used for cycling. Out tanks are largely a melange of many different bacteria. Laboratories take many, many precautions when they grow bacteria to prevent cross-contamination; the average fishtank wouldn't come anywhere near preventing other bacteria from growing.
It is a good idea, but probably infeasible with common at-home equipment. What exactly is your goal? For most common use, just using filter floss from an established tank or seeding a new filter by running it in an established tank for a month is good enough. It's technically illegal for you to sell your own bio-spira because Marineland Labs owns the patent on the cycling bacteria. I know it seems weird to be able to own a patent on a living creature, but that's the state of the legal system today. That's why the other successful cycling product available in Europe, bactinettes, aren't available here. Marineland won't let any other product with the proper cycling bacteria be sold here in the U.S.
