There are no literal references to palliative sedation in the Bible. The humiliation of man, as manifested on the deathbed, is again and again a sign of our broken existence. How good it was at first in Paradise, but look what we have made of it.
If the Bible notion of mercy applies anywhere, then certainly with a person in the extremity of his existence. Relieving suffering at such a time is not an option, it is even a command. But in such a way that dying is not (intentionally) accelerated. In that sense, the example of offering a cup of cold water is more than just an illustration!
Also the Lord Jesus, just before dying on the cross, took a sip of vinegar from a sponge which the Romans had tied to a stick by means of a stem of hyssop (in those days vinegar was a comforting beverage). The use of sedatives cannot be found literally in the Bible. David does however make use of his harp music to sooth Saul.