If you have the ability and experience to machine the headtube, you should be able to best answer this by yourself taking a few simple measurements. If you can't make this call by yourself, then you shouldn't be attempting it.
I modified a carbon road frame from an english threaded bottom bracket, to PF30 on a milling machine, and it works perfectly fine. No different than a factory designed frame. It didn't require multiple internet opinions.
It's either possible or it isn't. A dial caliper doesn't lie.
I would not be so concerned that the BB shell fails....it usually does not lead to the same kind of catastrophic failures that a headtube can cause.
I crashed hard enough on my Tallboy to break my leg,
The carbon is way more durable than most people.