1. You can quilt it in sections on the machine—maybe thirds that will fit through the machine without a struggle. Ann Peterson, Leah Day, and Marti Mitchell all have tutorials.
2. You can tie from the back as Kalama suggests.
3. You can use an invisible stitch, also called international stitch or hidden tie, what bearisgrey speculates on. Use either baby yarn or perle cotton and a doll needle. The outside shows a single stitch and all the other thread is in the batting. Here is a tutorial.
https://youtu.be/cEkHht2oJsg