I love your quilt! I'm saving the picture to my "Inspirations" album (with link) because it's just the type of work I enjoy seeing and would love to have on a wall.
I think your idea of random rather than continuous motifs is the best choice for this project, both in style and execution. You could put motifs in the blocks wherever needed to embellish and/or fasten down, even if just a few random running lines or tiny motif in a larger blank area. If you run into a problem area that's difficult to get a needle through, just change to a different spot--using separate motifs, exact placement won't matter as long as you have balance overall. And the layout of your blocks tells me you know all about visual balance. Beautiful so far, I look forward to seeing the finished product.