From the default Compose file, it seems that qt5 input ignores quite a number 
of the dead keys used there*:
 + dead_belowbreve
 + dead_belowcircumflex
 + dead_belowcomma
 + dead_belowdiaeresis
 + dead_belowmacron
 + dead_belowring
 + dead_belowtilde
 + dead_currency
 + dead_doublegrave
 + dead_greek
 + dead_invertedbreve
 + dead_psili
 + dead_stroke
This seems to be due to the missing definition of the corresponding qt 
constants (see: src/corelib/global/qnamespace.h:823-842 
 src/plugins/platforminputcontexts/compose/qcomposeplatforminputcontext.cpp:65-86
 src/plugins/platforms/xcb/qxcbkeyboard.cpp:412-431)