I just realized from Robert I's post that I misunderstood Andrew's question:
"Would the dried vinegar/steel solution react at all with the epoxy?"
My response that: "I don't think the solution would work through the epoxy" was because I thought that you wanted to stain through epoxy already on the boat.

I was concerned about the effect of the vinegar on the epoxy and ran some tests last year. The MAS epoxy I use set up properly on ebonized cedar. I tried a couple of treatments after treating the wood, including simply rinsing thoroughly with water and also using a baking soda (Sodium bicarb) solution followed by a water rinse. The water alone seemed sufficient so that was what I used when building the Guillemot.

My concern now is what affect the black color is going to have on the epoxy. It's obviously going to get warmer in the sun than the lighter areas so I'm wondering if it will get hotter than the epoxy can tolerate. It may be a better idea to use the ebonizing solution on the hull rather than the deck as I have. Time will tell...

Dwight