Is it permissible for a man, who has some confidence issues about his hair, to get a hair transplant on the part of his head where hair has never grown in the first place?
Bismihi ta'ala
Yes, it is permissible to get a hair transplant, as the hair will become a part of the body, and it will not create a barrier or an obstacle to prevent water from getting to your skin while doing wudhu'/ghusl.
The problem could be if the hair does not grow, and whether this would become an obstacle in ghusl and wudhu' being valid. For this case you must refer to your Marja' taqleed.
And Allah knows best.