"the install_name_tool lets you point a shared library at a different dependent shared library than the one it was originally compiled to link against"

Yes, but I'm not sure how we can take advantage of that.  First, keep in mind that there is only one version of _tkinter.so per Python installation and that is used by all tkinter applications, not just IDLE.  Second, the install_name_tool is not part of the base OS X system; it is included with either Xcode.app or the Command Line Tools package.  So we can't assume it will be available on users' systems.  And, third, changing the Tk library path may still break other third-party Python packages that also link directly with Tk and expect it to be in {/System}/Library/Frameworks.

"Secondly, we may want to consider adding something post-install (in IDLE?) where it would detect at least simpler cases of greatly out-of-date Tcl/Tk and offer to download and install a new one." 

That could be a useful addition and one that we could probably do in the context of a maintenance release.  The main problem I see with that is what to offer to install.  Since there is no truly open-source compatible option, I think we would need to provide our own Tcl/Tk 8.5.x package to install.  It's something to consider.