Then the question is when the movement took hold in the US (probably early on, I suspect) and what sort of Quaker 'activism' there may have been with respect to the Civil War.

I'm not sure that you would describe the movement as "catching on in the U.S." since the state of Pennsylvania was founded by (and named after) the Quaker William Penn. So in that respect the nation was born with a very large Quaker constituency. Quakers were and are opposed to violence and the term "Quaker guns" no doubt was used as a bit of irony. You are correct that their opposition to slavery put the Quakers into a delimma when it came to taking up arms against slavery. Many of them were conscientious objectors. President Lincoln wrote a Quaker in 1863

"Your people—the Friends—have had and are having a very great trial. On principle, and faith, opposed to both war and oppression, they can only practically oppose oppression by war. In this hard dilemma some have chosen one horn and some the other. For those appealing to me on conscientious grounds, I have done, and shall do, the best I could and can, in my own conscience, under my oath to the law. That you believe this, I doubt not; and, believing it, I shall still receive, for our country and myself, your earnest prayers to our Father in Heaven.

see http://www2.gol.com/users/quakers/abraham_lincoln.htm