I try to test build different x11-libs/qt-* packages with multilib portage and there, i found a problem, which prevented the installation of 32bit libs: The configure script does assign the right value to $QT_INSTALL_LIBS (/usr/lib32/qt4 for 32bit phase), but does not deliver it to the files, where qmake does need this value. In those files, the var is still there (lines like QMAKE_LIBDIR_QT = $$[QT_INSTALL_LIBS]). Either this var is undefined during qmake call or it is set wrong, but qmake does assign /usr/lib64/qt4 to QMAKE_LIBDIR_QT, which changes the install libdir (lib64 instead of lib32).
Do you already have a 64-bit version of qt4 installed? Which options do you run the 32-bit configure with? Is it working for qt-core?
Could you please attach "${S}/src/corelib/global/qconfig.cpp" from x11-libs/qt-core after the configure has finished? (i.e. after qmake has been built)
This has been fixed in qt4-build.eclass afaics. Anything else to do?
