First of all you misconcepted the meaning of promotion. Promotion is used for promoting the default widgt to a customized widget in designer. e.g. You have subclassed QLabel and created a class MyLabel. Then while using MyLabel in designer you need to place a QLabel and promote it to MyLabel. There is no meaning of promoting it to QLabel because its a QLabel already.

Secondly, whats the problem in subclassing QMenubar ? Have you tried that or what ?