Idea:

• Lasso a entity
• Drag it next to the car
• Right click on the car with the lasso
• Entity rides car
• Right click on the car with a lasso
• One of the entities dismounts and is lassoed (ideally)

By "lasso" I presume you mean a "lead" that is sometimes called a "leash" to reduce confusion with the metal that's spelled the same, rather than requiring an XU2 golden lasso. However, I hope it will also be possible to use mob transportation items from other mods, such as said golden lasso or TE morbs, to get the entities onto the livestock cars.

The unload feature could be a button for left or right dismounting of stock, no need for a lead. However, on my route, I would consider going to more realism with a Livestock loader/unloader example: https://s3.amazonaws.com/fallingrain/uploads/b2726e212e22428b8362ff634734748a/large/Kurraglen%20Cattle%20Loading%20Ramp.jpg?1330056236