I'm currently looking forward to going to the blockbau workshop at the end of June. I'm concerned that I will be stopped crossing into the US from Canada at the border with officials thinking I'm crossing looking for work. I'll be traveling with an American who should have no such issues. Does anyone have any advice or experience with this?

Why yes, yes I do.
You will be fine. Tell them your going to a timber framing seminar. In the unlikely event that they give you grief bring along some documentation that references the workshop to show them.
Pro tip here, don't use the word "school". I did on one trip to Heartwood and was hauled in the building and held up for several minutes while they tried to figure out how I can be going to a school and not have any student docs.