I think it would be fine. If you aren't sure, then make a throw using the velveteen as a sashing and see how you like it. The issue you might have is that velveteen tends to be stretchy. You might need to back it with some type of interfacing, maybe a wash-away one to stabilize long enough to get it all sewn together, or maybe do a quilt as you go so that you don't end up fighting that stretch. Pellon has some really lightweight stabilizers that are available at JoAnn or Hancock Fabrics.