Your work is truly amazing. I think quilting it on a longarm might distract from your work. If this were my quilt I would make sure that it is basted really well and then stitch in the ditch with a good quality clear thread. That might be the only way the quilting won't distract from the face. It looks like your rows are even so you could quilt in the ditch in rows. It might be possible to stitch in the ditch with a longarm and rulers. I can't wait to see what you come up with next.