I am very very fond of SITD...however, for this quilt I think I would do concentric circles centered over the blocks with the motifs. It would look especially cool if you made some blocks come forward and others recede by having the "forward" ones have complete circles, and the "behind" ones have circles that look like they are going under the complete circles. I don't have a useful drawing program, so I can't draw it for you (though it is very easily drawn) but
this is sort of what I mean, except that some of the circles would be complete, while others would be semicircles...rather than all being the same.
It is a beautiful quilt, and will be lovely whatever you choose to do!
Alison