Ah… ImageDraw.Point’s notion of x and y is different from the coordinates ImageTransformations.warp accepts. So by flipping x and y I got it to work:
iiw = round.(Int, reverse(invt(reverse(i)))) # inverse transform the lightning-rod coordinate as well (convert to Int too)
draw!(imgw, Cross(Point(iiw...), 10), RGB{N0f8}(0,0,1)) # draw that same location again?
