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?
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There is not a single accepted, standard way to do things like this in English. Nor do I think there is a particularly good way to do this.
#ed and #'ed both look OK to me, though neither is great. Hashtagged is probably your best choice.
The problem with "#ed" is that it appears to indicate the hashtag "ed". I'd never assume it meant hashtagged, and would try to interpret is as referring to editing, which is often abbreviated "ed."
If you insist on using a hashtag to do it, the only unambiguous way is "#hashtagged".
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.