Safety of zener diode based non isolated power supplies
Absolutely not.
In many locales, non-grounded and non-isolated electrical devices are required to be double-insulated.  An exposed touch pad would probably fail the double-insulation requirement.
Further, alternating current can flow through a series capacitor.  It doesn't matter what the output voltage of your non-isolated supply is; because it's non-isolated, it's always referenced to mains voltage, and there is always a direct path to mains voltage on every half-cycle (which is what makes non-isolated circuitry so dangerous).  A capacitive touch pad could couple this mains path directly to the person touching it, leading to a possibly lethal situation, regardless of whether the power supply is regulating correctly.
Use an isolated supply or use a button instead of a touch pad.