Valve does not typically make exceptions for taunts to load specific items you otherwise don't have, or change what the model is on the fly. Either it must use the equipped weapon, or a special prop must be provided when doing taunts like this.

custom bottle that has a handshake item but is the exact same model as norm scrumpy

Agreed, personally I think in a taunt where everyone is drinking, Demoman should one-up them by chugging TWO drinks at once (whether it be 2 bottles, 2 flasks, or even a bottle AND a flask)