I got this idea by crossdev, that creates a file with the emerge --info data when building so that it can be easily attached to a bug, even if it's variable. It should probably be optional and turned off by default since it would take time to save the files, and it would increase the number of files altogether.
The option could be a tri-state of "never", "on-failure", and "always", if you want to support testers who need to report that a particular combination works for them, and let the default be "on-failure" since typical users will not report successful builds. If the file is written under $T on failure, then it would automatically be cleaned up if the user deletes the whole working area, which is standard practice for a failed build that either has been reported, or will not be reported. Users who set it to "always" could have it written somewhere else, or perhaps even just included directly in the existing build log. Putting it inline is a bit unclean, but build logs are presently free form, so no tools should be seriously broken if this is added.
I definitely like Kevin's idea of having it a three-state and using ${T} for on-failure. Inlining it might be a bit too much maybe, but that would also work for me (even better one less file to attach on my bugs :P — on the other hand, bigger files, which might get rejected :()