I've done it for tied quilts, but not on a longarm. I like to put the batting on the bottom, then put the backing right side up, then put the quilt top right side down. Then I sew around the edge with my walking foot on my domestic machine. Leave about 12" to 14" opening. Turn it right side out. Then top-stitch about 1/4" to 1/2" away from the edge. Then you can quilt it or tie it.
You have to think about how you will handle the edges while machine quilting.
Another option is to quilt it normally on your longarm, then finish it with a facing, or by hand with a Knife-edge finish.