At this time, according to the manpage `slock` can only run a command when it locks the screen. So, there is no way to get the mouse reenabled (using just slock).
You can listen to events though, and that depends on your graphics environment. Check this question: Run script on screen lock/unlock, it should point to the right direction.
Note that just disabling the mouse with xinput can be trivially overriden by unplugging and reconnecting the mouse.