After contacting Kasikorn and SCB helplines today (and Krungsri branch staff previously) the concensus is that Thailand has no TOD (transfer on death) or beneficiary assignments to bank accounts. Interesting to hear of other experiences such as @gomangosteen who says he set a beneficiary with Kasikorn. It seems @freeworld has it right, as I was told by the two helplines that even joint account owners do not become owners in the case of the other owner passing.
It seems the last will is indeed the best way to go, even for ‘simple’ bank accounts! Live long and prosper, but be prepared… right?
Edit: Kasikorn says that both joint account owners must be present to make passbook counter withdrawals. Krungsri says either owner may do so. TIT!