Stephen Michael Hickey
I think the reason for there being no tie-breaker is that whatever they chose would feel arbitrary. You might use rongo tablets, of course, but there is no special merit in having most of them (not more than in having most workers or tribe tokens, for example).
The one thing in favour of using the Rongo tablets over anything else is that they are already used as a means of settling ties through out the game: in auctions. So it seems natural to use that at the end.
Using the number of workers as a secondary or condition to settle ties make sense too.
My gut feeling is that anything is better than nothing as a means of producing a result. Providing you know in advance what the end tie conditions are, you can alter your gameplay in your favour throughout the game.