It specifically is in the contract in the event assigned seating becomes a thing.

12.G.6.b "deadheading Pilots will not be assigned a middle seat, a seat in the last row of the aircraft, or a non-premium seat, unless these seats are the only types available yadda yadda yadda"

Obviously last minute reroutes could be an issue. But even OALs allow for a middle seat to be assigned (at least United did, not sure if it changed in this new contract) for $100 I think.