Dont get you. Most oil based will skin over after a while but you just need to burst the skin and the paint underneath is fine. Just make sure the lid is on proper and it will slow the process. Emulsion should last for a good while doesnt skin just goes off.
I personally don't think I would buy it as I don't find any problem with doing as JRS says. I just run a putty knife around the skin and lift it off. I usually decant it into a smaller container too using a pair of Oh's old stockings to get any tiny bits out. I think that if there was something that would stop it skinning over in the tin then it would probably stop it from drying properly as well when it was on whatever you were painting.
I dont think there is much need or demand for it. Its not a must have really as long as you put the lid on its fine. Plus if there was such a thing it would need to be far cheaper than a tin of paint or it would defeat the purpose.