MattiasOfTheMetal:
I see what you mean. Better to have tried and failed than to have wondered what if.I think I may default to using a stereo cable instead of mono. I'm trying to make something that would be easy to incorporate into guitars without much circuitry. Thought I'd try if it would just be something simple like a cap in line, you know?
Well there are ways to do what want, if the load was a constant or near resistance/impeadance it would be easier. However, as mentioned the easiest solution would be to keep the power an AC signal on separate wires.
Edit: Yet, if you still want to overlay AC on the power line, perhaps this filter tutorial will be more helpful. It's more in depth and much better presented than what I've discussed.
MattiasOfTheMetal:
But as for the stereo cable, I just need to make a sensing circuit for the pedalboard so it will stop supplying 9v power to the 2nd lead on the cable if a mono is plugged in. In this way, if the 1/4" stereo plug is used with a mono cable, the 2nd lug will connect to ground. I haven't decided on which method I may use: Use arduino to sense current, resistance, or voltage(I think voltage may be the easiest), or to just use a relay that will switch itself given the ground signal returns 9v, and then it will sustain itself there, then when powered off it will reset itself, or along the lines of that..
Can't really think of much else than that.
Technically an Arduino can only measure voltage directly, but current and resistance can be determined through Ohm's Law. So yes if you use an Arduino to detect the signal it's easiest to use voltage.
However, why not just use a multi-wire cable with discrete pins arranged so it can’t be connected incorrectly, as I’ve discussed previously? It doesn’t have to be a MIDI cable, I just thought that would be one of the easiest to find a replacement for given your situation.
MattiasOfTheMetal:
But if I used the arduino's analog input to determine the voltage(with a voltage divider of course), would the AC signal hurt the input if only for a short moment? I would like to not use an inductor if possible, unwanted distortion and low freq. loss through the arduino to ground.
First, all Arduino I/O pins are only rated to withstand about -0.5 V, so that is the hard limit for negative input. Second, I'm not quite sure what the signal is now, the 1 VAC overlayed on 9 VDC or just the 1 VAC.
MattiasOfTheMetal:
But onto the communication/wireless. I picked up 4 nrf24l01+'s on ebay for <$8, so my evil idea is to communicate the ardy's with a pair(1 on pedalboard, 1 on guitar) and then transmit that directly on the audio wire rather than air. I just wonder, from the theory you have taught me thus far, I could run the 2.4ghz signal over the audio lines and the +9v line, without issue, right? Or would I be better off running it on the ground line?
OK, I try to be very careful about making absolutist statements, but I want to make this clear it’s NEVER a good idea to put an AC signal on what the rest of the circuit uses as a ground! Ground is usually the shared reference, so if its voltage level is changing it can affect everything else that goes on in the circuit. Also, because voltage is an innately relative concept (i.e. it’s the difference in electrical potential of two points), under some circumstances it can damage logic ICs, like microprocessors. For example, the maximum supply voltage for an ATmega328P used on the Uno is +6 VDC, so you have a supply that’s giving it a very stable 5.2 DC, or 5.2 Volts above ground. Just a 1 VAC signal on ground will have the resulting power effectively swing between 4.2 VDC (when the AC is at +1 V) and 6.2 (when the AC is at -1 V), not because the supply’s output change but because the ground changed! What’s more a voltage regulator won’t help, because the voltage regulator will be using the same ground as the rest of the circuit.
If you want to run the signal on a cable without any issues, the best way is for it to have its own wire. 
MattiasOfTheMetal:
Now for the possibly crazy idea. For wireless, run that into another pair of nrf24l01+'s, but with an antenna. I don't know how I would go about amplifying that signal just yet though. Off to google I go!
Well there are quite a few options for wireless communication using Arduinos. Notice that if you scroll down a little, the nRF24L01+ radios have two entries… 