Hey! so there is a workaround to doing this but it's a little backwards, and will work only on all the loop tracks at once (so if you wanna mute just one track you'll have to settle for the mute option really.) what you wanna do is, while your loop is running, press MIX, select LOOPS on the first screen and then use the slider to change the volume. If you do a quick "flick" style swipe, it will automatically and gradually change the volume up or down, or if you prefer you can do a normal slide to control the speed but it will be more "bumpy" if you get me.
Anyway if you just wanna fade all loop tracks in or out while playing, thats your best bet