I also just played around with QMAKE_EXTRA_TARGETS to solve the question, but could not manage to get it done ;)
One other (simple) solution which might work for you is to wrap the call to gcc/g++: in the .pro file, set QMAKE_CXX=./g++Wrapper and in the g++Wrapper shell script, call the original compiler while doing anything you want before and after the call:
#!/bin/bash
DoWhateverYouWantBeforeCompilation
g++ $*
DoWhateverYouWantAfterCompilation
By evaluating the command line parameters, you could also restrict your actions to specific files.