Thanks to Drs. Lusthaus and Huntington for the useful replies.
I was not aware of the *Liudu jijing* parallel, and that is very interesting. My initial impression of Kumāralāta's work was that many of his stories were original pieces of fiction, but in the process of going through them I have come to realize that many must indeed be literary reworkings of older stories (just not famous stories, so it seems). I would say, however, that if he chose to keep this one element in the story, it may have have resulted culturally familiar to him.
In the course of the afternoon I did find a very relevant passage in Stein (Innermost Asia, II, p. 646) on Byzantine and Sasanian coins from the Astāna cemetery in Turfan:
"Mashik claimed the distinction of having been the first to learn by experience to look for coins of gold or silver placed in the mouths of the dead, though his search was but rarely rewarded. That earlier pillagers had not made the same discovery was proved by the fact that in none of the tombs which we explored, and which Mashik stated that he had not himself touched, had the skulls suffered the rude operaation by which he was wont to ascertain whether they contained a coin.
The fact that out of the four coins actually found by us in the mouths of Astāna corpses three are Byzantine gold pieces or imitations of such pieces (Ast. i. 3. 023 ; 5. o8 ; 6. 03) and one a Sasanian silver coin (Ast. v. 2. 02) might naturally predispose us to connect this practice with the ancient Greek custom of placing a coin between the lips of the dead as the fare due to Charon, the ferryman of Hades. But the reference with which M. Chavannes kindly supplied me in 1916 to a Buddhist story in the Chinese Tripitaka suggests that the custom was not unknown in the Far East also [4]"
The note is a reference to Chavannes, Cinq cents contes et apologues extraits du Tripiṭaka chinois, I. p. 248, which is a translation of the passage Dr. Lusthaus brought into the discussion. In what follows to what I have quoted, Stein seems to be also a bit puzzled as to the western connections of the practice. In any case the it is attested in Sasanian Iran, and in all likelihood it is more closely connected with Sasanian trade than with remote leftovers of Greek influence in Bactria. The question remains, though, as to whether it is attested anywhere else in ancient India or not.
I will check Tusa's work, and thanks also for the fascinating account of the Vietnamese funeral!
namaskaromi,
Diego