I considered the forces. Caliper is behind fork leg getting forced against it when brakes applied. Still not entirely sure what I will go with. See how I feel when it's done.
When you considered it, did you work out exactly how much torque is being applied, how strong a helicoil is and how well the insert will be seated? Like the Yamaha engineers did when they designed the calliper? There is no bullshit with engineering. It's pure science. Over the last 200 years every metal and every alloy of metals has been tested, examined under x-rays, improved and catalogued and if you look on the internet there are thousands of tables of precisely how strong each is for a given thickness, and how strong it is at every given temperature from -50 to its melting point. Yamaha engineers then design a part using finite element analysis and a back catalogue of tens of thousands of design tests.
And you want to put a helical in it and expect it to work on the basis of what? Have you even thought that brake callipers get very hot when they are working. How does that affect a helicoil insert?
And if you do it, how are you going to test it for strength afterwards?
I'm not saying it won't or can't work, what I'm saying is that it sounds like you are not in any way qualified to make that judgement, hell it might be fine, but ask a professional before you do it, I'm begging you. I'm in no position either, if it makes you feel any better, but I'd ask someone first.
You would be right to think I'm writing a very strongly worded post here, but braking from 100mph into a left handed hairpin bend on a 500lb bike is not when you want to find out you got it wrong!