Not being a developer, I suspect I'm just talking gibberish... But why is it an all or nothing proposition?
Wouldn't it be possible to maintain a reference implementation of the Linux specific calls, which otherwise "should" be in libc? Since application developers would need to do something anyway (writing code to use the new calls), having +1 runtime .so seems to be all but insignificant.
Some libc implementors might ignore this, some may copy individual functions verbatim into their system, some might link to it, and put a thin layer over top. Some (say, those who disagree how memory copying should work....) could just use it for reference, to build compatible interfaces with entirely different internals.
It might also be possible, with this layer, to give post implementation stability, additional time for interface stability. That is, while the kernel syscall() might "never" change, the interface provided by the reference implementation has 12 months, or 2 releases, or whatever, to stabilize.