I strongly believe that technically the present Dalai Lama has the "final" say in this issue. if he officially says there will be no further Dalai Lama, then it should be final and any Dalai Lama chosen by whoever and by whatever means would not be recognize by Tibetan Buddhism.
However, there can be exceptions. If the Dalai Lama do not officially announce his decision, then one can always argue that the Dalai Lama did not actually say it and proceed to chose a new Dalai Lama. Then is the Lamas in India does the same, then chaos will ensue.
Also, even if the Dalai Lama announce officially his decision that this will be the last incarnation, still there can be some way for any party, after the Dalai Lama's passing, announce a certain high lama has a dream or was told by the Dalai Lama to chose a new Dalai Lama.
From these scenarios, it seem to me that the only way to avoid chaos is that the Dalai Lama announce, he will incarnate in Tibet and let the Chinese choose the new incarnation and instruct the exiled Lamas to accept this. This is the only logical way to avoid chaos. Strange scenario but true.
What do everyone think?