Was the addition of diphenhydramine in textbook quantity to potentiate the opiate or was it added in quantity for it's own therapeutic effects? If it's the former then I think that such an elixir may be unique to India as I have no knowledge of any company making that concoction with that intention in mind. Do you remember the name of the company that makes it? I'd like to see their research documents if I can.
Personally I found the potentiation quantity of diphenhydramine to be far less powerful than a simple therapeutic quantity of dextromethorphan; however a high therapeutic dose of diphenhydramine has no discernible effect on me.