I do agree with the consensus that the reading of your great-great aunt's name is most likely Chisako.
I could not find any chi/sa+咲子 on Japanese passenger lists on FamilySearch, so I had to make do with googling chi/sa+咲子 combinations. Regarding sa+, there are no results that include readings (and very few results that include just the kanji), so in itself, I would rule out any Sa#ko readings for 散咲子. As for chi+, the search results I found reveal Chisako is the only reading that shows up at all.
My PNL:
http://www.behindthename.com/pnl/149249'Maybe It Is Daijiro (aka Maks)' blog:
https://maybeitisdaijirou.wordpress.com/'Beyond Sakura and Hiroshi' blog:
https://beyondsakuraandhiroshi.wordpress.com/My Twitter: @maybeitsdaijiro