Should I include my (unofficial) class rank in my SOP?
In general, I think mentioning unofficial rank is fine. You could simply write:


  Note, though my university doesn't officially publish class ranks, I learned that my 3.6 GPA corresponds to the 94th percentile.


The only trouble is that they might not believe you; it would be better if you could get a professor to mention this fact in their letter. 

Your case is a bit different, though. You don't know what your final class rank is (even unofficially); all you know is that two years ago, you were in the 94th percentile of a subset of the students after weighting for the number of credits completed. There is no concise, accurate way to describe this that doesn't appear desperate*, and you wouldn't want to be accused of trying to mislead them. So, I would skip mentioning this altogether.

*Actually, the one way I can think of is maybe you could present it as an award? "I was ranked in the top 6% of applicants and was therefore allowed to study abroad in Jamaica?" If study abroad is a competitive process that you "won", it may be fair to mention this.