You may be able to block it. I have successfully blocked quilts with bowed borders. I have a design wall, but you could do this on a carpeted floor or head to a home supply store to purchase 1 or 2 sheet of insulation foam (it comes in 4' x 8' sheets.
i put a white sheet on my design wall and then use a water soluble marker to mark a square/ rectangle that is larger than the quilt. Use your biggest square ruler to make sure all 4 corners are true 90degree angles.
then lay your quilt outn your prepared surface, pinning it as you go and using the square you marked as a rough guideline. Next spray water on the quilt until it is damp all the way through-- you don't need to get it sopping wet.
once it is damp begin to gently stretch it toward the edges that you marked and pin it to the surface as you go. You may need to go around the perimeter of the quilt several times before you get it good and square. Let it dry overnight.
hope that helps.
Rob