To answer the original question, in theory you should not have to do anything in emulators to display the proper image (that is, if they supported it). Most OSes are capable of identifying the attached monitor and expose its complete specs so user applications can deduce which transformations to apply to the image to make it look as if it was displayed on a CRT TV or monitor.
Note that this would look much better on a 4K+ monitor as this would allow to allocate several pixels of the screen for each emulated CRT pixel, thus avoiding annoying fringing issues which occur when stretching an image using a non integer factor.
I have used FS UAE on my 5K iMac and regardless of the window size I must say that it is extremely hard to notice any fringing or other scaling artifacts. Alas, FS UAE does not take advantage of the monitor spec and I would have to adjust the image size manually to make sure it displays properly proportionned pixels.
Now the question becomes: who will write this code?
