July 25, 2016: First off, I finally got the design back to where I wanted it to be to stock it on Tindie, so there's a handful in stock. I plan to build more soon, because those first ones are probably going to sell out quickly.
Second, I was able to buy inexpensive flexible Kapton circuits from Itead, and it looks like they did a great job building them. I only built one so far, and it works great. I will be building more soon for mounting in tires and balls.
Speaking of tires and balls, I tried two different types of tags in the first tires and balls. When I got the samples back, one type of tag had died due to the vulcanization process. In both cases, it appeared as though the "gold" pads on the Kapton had corroded. I suspect what actually happened was that the gold plating didn't completely cover the copper, and the copper corroded due to the heat and bloomed through the gold, breaking the trace. These traces were the ones that go to the antenna, so when they broke, the sensor became unresponsive. The type of tag that survived was not a sensor tag - it just served a URL. The fact that it survived is good because it tells me that sensors can survive the vulcanization process, but it's bad because these tags aren't sensors, so their survival doesn't tell me as much as it might have if they were sensors.
So now the most recent version of sensors (that still have a rigid board) are getting ready for mounting into footballs. I just ordered a solder stencil for the flex circuit array so I can make those 8 at a time. It should be here within a week or so and I'll knock out a couple dozen of the flexible circuits for more mounting into tires and balls.
Hopefully I'll be having these produced at a contract manufacturer instead of hand assembling them. It will take a fairly big commitment on my part to ramp up production though. I'm not sure I can sell enough to pay for that. I'm working to do some customer discovery to understand my value proposition and so on to determine if this is something that I can sell in any sort of quantity. I'll keep you posted.