I saw a picture of one of these with the outside unit installed between the chassis rails. Sorry... I cannot find that image now.
I am no fridge mech, but I would think that the evaporator could be in any orientation. The compressor may have to be relocated and kept vertical, but I don't know if this is mandatory or not.
Sadly, I have filled all of the available space between my chassis rails, or I would have probably tried to install one of these units there.
(Edit: I am talking about one of the Mitsubishi Heavy Industries domestic split systems)