I was in need to use multisampling for off-screen pixel buffers WITHOUT render_texture extension in my application on Windows and was unable to do that with Qt. My graphics card has support for this feature and I already tested it with a pure OpenGL sample.
I was curious to know why it was not working with Qt so that I found the cause in the Qt source file qglpixelbuffer_win.cpp. There there's a commented section saying:

// sample buffers doesn't work in conjunction with the render_texture extension
// so igonre that for now

But, if that's true, why not, instead of disabling multisampling with pixel buffers at all just because of the render_texture extension, make the features work mutually exclusive. When the user wants to use multisampling, render to texture becomes disabled (just use a multisampled off-screen pixel buffer), when the user does not want to use multisampling, render to texture becomes enabled, which means, to use render to texture, not using sample buffers is a requiriment.

See attachment for a diff of a patch I applied locally.