You don't want to create an actual placeable model. You want a regular creature model, just full body (assuming it is a creature with a separate head). Then you just add a new row to placeables.2da and specify your creature model. Look at some of the vanilla "siiters" for examples of how to do it (rows 199-212).
In order to get it to animate, you'll need to add a an ANIMLOOP animation to it. The existing supermodels only have sitting, sitting & drinking, and sitting & playing Pazaak (ANIMLOOP01 through 03, respectively). You'll need to add a copy of whatever animation you want to use onto your model and name it ANIMLOOPXX (available numbers are 04-10). You should then be able to set the animation state in the UTP and/or script it.
Alternatively, you could stick with a regular creature and rely on editing the level model in order to create a suitable walkmesh to allow it to lie on a bed. The Ebon Hawk module does this, for example, in order to place the player and Visas on the med bay bed in cutscenes in TSL.