I wonder why its so hard? I once built a modern gcc + glibc in my home directory on an older linux (to be able to run modern programs), and it was mostly "relocatable" or "portable" (what I mean is that you could copy it around and run it from other directories).

Couldn't somebody build a full cross-compiler toolchain as "relocatable" binaries, depending on an older kernel, and then just offer that as a binary download to run on most recent distributions? It's not a typical way to distribute a linux application, but it should work in principle.