I noticed that when sling loading was first introduced in the dev build and if I wanted to say have a certain vehicle to be slingloaded I had to put in a command in it's init field: unitname setMass [10,0.5].
Using the setMass command in the init field allowed for you to pretty much be able to sling any type of vehicle (which would be good for mission makers).
There is no change in the behaviour of this scripting command or the sling loading feature.
Except the mass, vehicle needs to have set proper memorypoints and parameters in model/config as well. If you change the mass for a vehicle, which has these parameters properly set, it works fine. If you change the mass for a vehicle, which is not prepared to be sling loaded (=there are not memorypoints or/and paremeters in config), you won't be able to sling load it. It works this way from the beginning.