It looks like #ed would be correct way to write word "hashtagged", but this can be confused with hashtag – #ed. What would be another acceptable way to write word "hashtagged" using # symbol?
The goal is to write word "hashtagged" using # symbol unambiguously. Then the reader would benefit by seeing a # within the word suggesting the practice to some other word that might be hash-tagged [Spell Check demands a hyphen tribute].
I would recommend the use of "ha##tagged" or "hashta##ed" as they would get accross the idea of how and where #'s could be used to mask letters while passing the word along.