Here’s a suggestion, not sure if it will work, but it might be heading in the right direction.
Assign the SSID to a VLAN
Have a Layer 3 interface for the VLAN on the MX64
Configure a source-based default route to your server
Note: you need MX15.4 or later, your server needs to exist in a subnet that is configured on the MX (or connected to another MX, which is connected via AutoVPN), and it won’t work if there is a specific route for the traffic on the MX, it’s only for default routed traffic (I.e. no known route).