If you configure your LIO with the 4-pin XLR output jack, you cannot use an adapter to convert to 1/4" (signle-ended, TRS).
In other words, you cannot tie the L- and R- terminals together to GND. This would short out the headphone amp's balanced output stage.
What you can do is order your LIO with the TRS jack, and when you get your LCD-3's later, I can send you the 4-pin XLR jack and cable that plugs into the HPA module. It is very easy to install (two philips head screws and a plug that plugs onto a header on the HPA module).
The HPA module has BOTH SE and BAL output signals. So you either tap the SE output, or the BAL output based on the jack that you use.
For LCD3s, I highly recommend the BAL output. It has more output voltage swing, which I believe you will enjoy with the LCD3.
All the best,