Angelina i wonder if more frequent use can cause physical harm?
Not harm in the way that something bad happens to the area where it is used.
BUT:
Sodium channels also exist at the heart and Lidocain has also been used as an anti-arrhythmic drug in resuscitation, but these days it has fallen out of favour for that use).
Depending on the actual type of local anaesthetic in the gel, the effect would last somewhere between 15 minutes and maybe two hours. So replacing a belt would require you to re-apply very frequently.
Besides:
Depending how sensitive the wearer is on various spots, it might not be 100% effective.
E. g. application to labia and clitoris still leaves the vagina available for stimulation inside. Not what you want if enforced chastity is the goal.