Please note that 1 level mixin is not sufficient in the case of libraries. In too many cases you will not inline the function that does the work, only the interface wrapper.
I don't think I would ever want to inline the whole call tree of a library. I've certainly never wanted to do anything like that in 20 years or so, and I've worked on some really performance critical systems, like amiga, dreamcast, ps2.
It still sounds really sketchy. If the function that does the work is a few levels deep, then there is probably a good reason for that. What if there's an error check that writes log output or something? Or some branch that leads to other uncommon paths?
I think you're making this problem up. Can you demonstrate where this has been a problem for you in the past?
The call tree would have to be so very particular for this to be appropriate, and then you say this is a library, which you have no control over... so the call tree is just perfect by chance? What if the library changes?