The libqcocoa.dylib plugin is created only as release, independent from the configuration (I run -developer-build). This setting is found in qtbase/src/plugins/platforms/cocoa/cocoa.pro, line 96 to 101.
The problem is however that Qt5 does never load the libqcocoa.dylib library when qt is used with debug libraries. The problem is that the library does not pass the test in qlibrary.cpp line 720, as debug is false.
I tested to change cocoa.pro to build both debug and release, and I did get a libqcocoa_debug.dylib. It does however still set the debug boolean to false on qlibrary.cpp line 704, and I was not able to figure out how to turn it off for the debug version of libqcocoa_debug.dylib.
My suggested solution would be to build libqcocoa in both debug and release, and have different replies on the debug-query for the two (as detected on qlibrary.cpp:704)