You might be wise to ask to see an example of your selected longarmers stitching in the ditch before you decide on that. Actually, it's always good to see an example of their work before agreeing to turn your quilt over, and to get the agreement in writing of course.
Several LAers that I know will do it if asked, but they hate it and aren't anywhere near as good at it as they are at other quilting methods, including custom freehand. The difficulty comes when the ditch side (low side) crosses to the other side at an intersection and the ditching is not adjusted quickly enough, or at all.
An 'inspiration quilt' that just used SITD wouldn't have any visible quilting, so make sure that's what you really want.