Current openMSX scaling code can't really handle that. It only allows multiples of 320x240 at the moment. So it's not as trivial as you may think... thanks for the idea though.
So, it's not possible to scale the openMSX window and the MSX screen independently?
Only window sizes of multiples of 320x240 are currently supported. Within the window you can only use horizontal stretch for some kind of scaling, but that's not what you meant, I guess.
I thought with the possibility that the openMSX window could be stretched while maintaining the MSX screen area.
As an example, my configured scale_factor is 4 with the SDLGL-PP renderer, this results in a MSX screen defined by a 1280x960 window. It would be great to allow resizing the openMSX window while maintaining this MSX screen size always multiple of 320x240.