Back in my motorcycle days, I'd fix seat tears using a curved carpet needle to sew it up, and then seal the sewed up area with black silicone sealant/glue. You could forgo the silicone if you didn't require waterproofing the repair.
It wasn't pretty, and on a bicycle saddle, the repair might rub you in the wrong place.
However, it wasn't a long lasting repair as the new holes made by the needle would start tearing over time.