Is there a way to do that already?
Actually there is no option to do that. But one possibility is to create custom dithering matrices with extensions. For example this extension has the same bayer matrices but with a zoom=200%. You can add them from Edit > Preferences > Extensions > Add Extension. You can use this .zip as an example to create your own matrices.
Actually, I'd never considered being able to adjust the pixel size of the dither, that would be interesting. I wonder what a gradient with a dither that gradually gets larger pixels as it gradients would look like?