Was there every a release of Rosegarden using the C++ Gtk wrapper? At least I don't remember such, and based on the linked post, it seemed more like some experimentation until concluding that the Gtk C++ wrappers were too immature.
The switch from the Motif was of course needed because that stuff was ancient already back then, but I had understood that it wasn't just UI toolkit switch, but more like a total rewrite of the application (and either can kill a project like almost happened with Rosegarden).
As to LyX, that is a good example. Regarding its multi-toolkit support, I think its Xforms and Qt UIs were the only ones that were properly supported. I don't remember the Gtk interface ever working properly, it was always lacking a lot of features.
Does LyX still have multi-toolkit support (can't check that now as LyX site doesn't seem to be responding) and are/were there any other as complex open source projects that support multiple toolkits?