Forum OpenACS Q&A: Response to Revisions to post by admin cause loss of originating author.

This functionality isn't there, and AFAIK you're the first person to ask for it.  So it's not surprising it's not there :)

It would be an interesting enhancement to the content repository (not just the bboard, I could see people asking for this for other content, too).  But given that you're the first person to ask for that in our community, at least, well ... you know how it goes.  Open Source, you may have to offer to look into what it would take to make such an enhancement yourself :)