Is it possible to hide your MAC address before connecting to open wifi?

You are going to want to research Public vs Private IP address.

Note: Before you connect to their network you have NO ip address(assuming you are not connected to another network at that time).

Let's say you are at a coffee shop and you want to connect to the Wifi. The coffee shop has a public ip address and when you connect to the Wifi hotspot, it is going to assign you a private IP address. This private IP address can be assigned to a different user later when you are done using it. So when it comes to hiding your identity before connecting to a Wifi hotspot you are going to want to change your MAC address also called MAC spoofing. (Note: MAC addresses work at Layer 2 and IP addresses work at Layer 3).

This is because some Wifi hotspots or businesses can keep track of which MAC address was assigned to which private IP address and since MAC addresses are unique to every device, they can use this to associate your computer being connected to that private IP at that time.

Also research Layer 2 vs Layer 3 in the TCP/IP hybrid model.

Hope this helps!