Just in case someone is interested, I've managed to solve the issue the following way:
Qt Code: 
  1. QMAKE_LFLAGS_PLUGIN += -Wl,-install_name,@executable_path/../Frameworks/$${TARGET}.$${QMAKE_EXTENSION_SHLIB}
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Unfortunately, in contrast to what QMAKE_LFLAGS_SONAME does for an ordinary library, this way the install_name doesn't get prefixed but is totally replaced, i.e. it is necessary to specify the target file name. I haven't found a way to do that, therefore $${TARGET}.$${QMAKE_EXTENSION_SHLIB} is used as a workaround. Let me know if there is a better way to do that.