Plasti-dip or equivalent would give you a more professional finish but as electric tape is designed to prevent contact with electricity you can expect the same degree of protection from the lead wire. As the lead is exposed only at the squared off end of the cut you will have not problem created an effective safety cap.
If you keep your phone in the front pocket of your pants your reproductive organs will be protected from the radiation 😉
Actually it was suggested by a customer to rig some fishing weights between the double layer of the inner wear sheath to gain more extension. Lead wire is used to fashion sinkers by fishermen and it was obvious as soon as I seen the coil on the shelf how it could be incorporated as a penis extension device that would work with the inner wear.
Plasti-dip was the first consideration but testing proved that the double walled, adhesive lined, poly tubing was cleaner, easier and more durable to produce than the dipped protective coating.