How to simulate sequential registers from causal ones?
Answer my own question: The answer is (probably; please let me know if I am wrong) negative: it is impossible to simulate sequential registers from causal registers. 

Note: the following proof is indirect. And I am still interested in a direct proof.


  Theorem: It is impossible to simulate sequential registers from causal registers.


Proof: In the thesis [1], Kawash shows that causal consistency memory (with only $\texttt{read/write}$ operations) cannot solve mutual exclusion. However, Lamport's bakery algorithm (wiki), when run on sequential consistency memories, solves mutual exclusion.



[1] Limitations and Capabilities of Weak Memory Consistency Systems by Jalal Y. Kawash, PhD Thesis, 2000.