I understand your rationale.
People don't hate hair transplants because they're seen as vain, no, everyone knows the importance of looks, of hair, even the people who have their heads up their asses know that they're spewing nonsense if they ever utter the phrase "It's the inside that counts!" You know this, they know this, everyone knows this.
No, the reason they hate is: you're improving yourself, people hate seeing others improve, and hair is a dramatic improvement on top of that, as we know that looks are king, and people know it. They would want you to accept your decay, so they can shine a little more next to you, or at least no feel so inferior in comparison. By elevating yourself, you're depriving them of that.
And if you get in trouble for having had a hair transplant, it will be for this reason, not because it is seen as vain.
A little lying? The only more elaborate lie I could think off would be wearing a hair piece or having a mistress. This is going to take a lot of energy and dedication, but if you think it's worth it, why not? I hope it all works out for you.
Telling the truth takes less energy, and I really don't think I'll get mocked and laughed into oblivion or fired for having a hair transplant.
I've dealt with colleagues having plastic surgery before. A female colleague got a gastric bypass, she didn't keep it a secret, and what happened is that the ugliest, bitterest b****s at the office started gossiping like crazy, and even attacking her to her face "why would you even do that, at your age, appearances don't matter to anyone! What a waste of money! You're irresponsible!" They were quite aggressive about it. Insecurity, as always. They quickly got tired of it, and now she looks great, from fat to totally bangable 40 year old MILF. No gossip anymore, only positive remarks.
I know no one wants to hear it here, but the best way to go about this is: tell the truth and own it.
Try to lie and to be sneaky about it and that's when the problems will start to escalate. If they suspect and discover you, they'll see you as a sneaky liar who thought no one would notice, and they would be right. By telling the truth, you disarm people somehow, or if they have something to say to you, it will out come out at once. And you can be like my female colleague: "OK, you're done?" and move on.
My 2 cents.