Pin 4 is ground, 16 is power, 9 is manufacturer's discretion. If the Porsche guys say they're getting rpm from 9, then that's what Porsche is using, but that doesn't nessecarily mean anyone else is, or anything plugging into the obd2 port is picking up on it besides that specific Porsche tool. If they even pick it up, I don't know. For scan tools and adapters, they get their info off the network, which is just data stream deciphered by the tool/adapter. You can't just throw an rpm signal into that circuit, it won't work. Now, there's always a way to do what you want, it's just a question of how far you want to take it lol... You could get a vehicle obd2 PCM, wire up power and ground, convert your 2 stroke rpm signal into something the PCM understands, then read that off the data line. You could also in theory use the PCM to accept cylinder head temps, water temps, charging voltages, etc and read those off the network. But if you're using an app you'd have all kinds of codes.
Thinking as I'm typing here, of course there's going to be a few things the PCM will need to operate, depending on the build of the vehicle. I'd choose an early model obd2, less complicated. Again, depending on the chosen donor vehicle, there's a chance the PCM needs to see other modules in order to function, so you'd need to know operation of the PCM and it's network in order to determine if it's a good donor or not. Many signals can be duplicated with fixed resistors, as long as you know what the values are supposed to be. Complicated yet? lol..
I haven't had breakfast yet, so the mind's a little weak still. This is possible, and I hope there's other ways to go about it, this is just the first thought that came to me.
http://en.m.wikipedia.org/wiki/OBD2This is the link you should be looking at, describes pin outs and stuff.
