The way I see it you'd have to regenerate the Makefile, for example like this:

qmake UICFLAGS=....
make -f Makefile

and in the project file add:

DEFINES += $UICFLAGS

Of course get rid of the -D prefix when filling the UIFCFLAGS variable.

An alternative way is to alter the way moc works:

new_moc.output = moc_${QMAKE_FILE_BASE}.cpp
new_moc.commands = moc ${QMAKE_FILE_NAME} -o ${QMAKE_FILE_OUT} ${QMAKE_CXX_RELEASE}
new_moc.depends = g++ -E -M ${QMAKE_FILE_NAME} | sed "s,^.*: ,,"
new_moc.input = HEADERS
QMAKE_EXTRA_UNIX_COMPILERS += new_moc