Have a livestock barn that opens up into a dialog and behaves as a paddock split up with internal fencing allowing moving around of livestock (see screenshot)
A barn is not much different to a paddock with electric fencing splitting it up. So you could have a livestock barn represented by an icon on the farm map. Then you could select a mob and move it to the livestock barn icon. When you hover over the livestock barn then a dialog pops up showing the barn, stock could then be dropped in a pen. Setting up the barn is in effect a map without any aerial images. You create the barn boundary (a paddock) and then use electric fence to split into pens. Water and gates can be added as you do on a paddock. You could then add bales of hay to the pens and look at LU per pen / bale. Stock can be moved around the pens as normal and even allow gates to left open etc.