(first-time poster, beware of possible stupidity) I'll throw in my views on the subject:
(1) If I understand correctly, SONAMEs are not meant to provide any other metadata than a reference to the *library's* ABI. Using SONAMEs for anything else, like which compiler the library was built with, will most probably result in very broken behavior, because the upstream authors have little way to ensure that their library with SONAME n will always be built with compiler x but their library with SONAME m will always be built with compiler y. (2) If (binary) programs from outside the distribution are to work with debian's libraries at all, the metadata about the compiler has to go somewhere. I'm not worried about the transition within debian (which can be some pain, too) but numerous third-party binaries that will probably break even though compatibility for them could have been retained. This rules out just replacing the old libraries with new ones compiled with the new compiler. (3) The easiest way to have metadata about the compiler version is using a separate directory. (4) If the libraries are linked against by their SONAMEs (making them indistinguishable), but the compiler version used in compiling the program is deducible, hacking the linker seems a plausible solution (akin to having two linkers in libc transition). Just my twopenny Panu