I would hint that while your excited for SIL's bash, for yourself you'd be much happier with something much more small and intimate. I'm sure there's a way you could bring it up tactfully. It would be less of a financial burden for your hostess and she might be relieved to learn that you are not expecting or wanting an extravaganza.
You could also ask DH to gently steer her in the right direction
At the end of the day, especially if it's a surprise, you really have no say. She is giving you the gift of a shower, and you must accept it graciously even if your staring out at a sea of people you don't adorned with a paper-plate bow hat.