I'd like to get to XingPing from Chengdu, is this possible via an overnight train?

I'm afraid there is no direct train between the two places, so you need transference. You can take the train from Chengdyu to Guilin first. It takes about 26 h, more than a whole day. Then you can go to local Railway Station to take the bus to reach your destination.
For time-saving, you can also take a flight from Chengdu to GL. It takes about 1.5-2 h.