The O.T. idea you talk about is a good place to start. A sensory integration assessment might be useful in more clearly identifying sensory needs in a number of areas. Though I disagree with the theoretical underpinnings of SI, I have seen it be successful. As I understand it, not all OT professionals have expertise in SI assessment and treatment. The other day an good OT asked me what I meant by sensory integration assessment. I would look for someone who doesn't ask that. Also, beware turf. Sometimes school OTs may not have SI expertise, but can get snitty if not at least consulted before a student has an SI assessment elsewhere. I would too.
There are SI oral stimulation techniques that may be found useful for you client.
Also, functional analysis may suggest other "causes". How do caregivers respond? Some fail to understand that hollering "Get out of that drywall," can be very reinforcing. DRO, DRI etc of course come to mind. Look closely at what this client is getting out of PICA. It may not even be true PICA. I think the definition says something like "compulsive eating of inedible objects". Look at what the behavior communicates. If behavior ananlysis is not your area, perhaps a behavior analyst could be consulted to futher elucidate the behaviors functions.