I have seen all kinds of felt and thicknesses. I would be worried about dye coming out. Check a piece before you use . It will only mean one quilt to undo. I was told I couldn't use an old blanket as batting. I did and put it in a show. The quiltilice never noticed. Only thing I could say the quilt was very heaven in comparison with one with batting.
Use the felt as applique for rabbits horses etc.