I'm not sure I see much good in integrating non-kernel stuff into the kernel tree, but I wonder if it would make more sense if some of these tools were perceived as test tools comprising part of a kernel test suite.
Then, there would be a test tool that exercises some part of the ABI while the user-friendly tool is hosted somewhere else. The test tool could be developed in-tree until the ABI is declared stable, at which point it is forked into an external project while the original tool can be used as a reference implementation for other tool developers.
When that "stable" point is reached may be contentious, but it may also be straightforward. For instance, the first patch that says "Make tool <foo> easier for users to run..." could be a good indicator that it's ready to be forked out.