- What is the significance of this signature (if any)?
- Is it in any way related to the seven kabbalistic seals?
- Are there other similarly unusual signatures by other prominent talmidei chachamim?
A friend on Facebook shared this answer which I have not sourced yet.
his signature is because his father's possessions were all lost in a ship wreck. He was thankful to Hashem for giving him kaparah taking his money and not his life. He had special dishes prepared with a sign of a boat and made a seudas hodah. He adopted a similar signature for his name. It's written up in the Chida's own sefer Maagal Tov
As a point of interest for #s 2 and 3 in fact there a similarly ornate signature which does contain some of the kabbalistic seals. It belongs to Shlomo Molcho