Sorry. While the OP gave a lower limit of no pen and paper, the upper limit of no FLARM was not mentioned.
I'd still use dedicated equipment even if disinterested in the five-lettered thingy. Using personal smart phones seems flawed. For example make sure other networks are not going to grab the wing runner's phone when the towplane is out of range. The significant other of the wing runner bringing lunch, or the wing runner's own logger being turned on could be that network.
Jim