Did you drill holes in the bottom for drainage, or will it be more like a bog when watered?

yep, holes drained

Heard of horsetail reed grown in a Plano stock tank ( 5th picture down). Though may be better options for El Paso.

What about a tecoma stans or Caesalpinia mexicana? They're both pretty and drought tolerant.

I second the Caesalpinia mexicana or my favorite Caesalpinia pulcherrima It should be something of a focal point next to the gate.

that might look nice; we have a lot of those around. Wouldn't that need full sun though?

I have a thriving Pride of Barbados that gets no more than 3-4 hours of sun. I live in an oak forest.

An agave won't need water and would look good all year long. Try to get a really striking one, like the ones that have yellow, or white or light silver. Use some type of gravel or rock around the plant to give it a polished finish. If the thorns at the tip of the leaves become a concern, you can tip them off periodically. Another option could be Hesperaloe. You could even plant different colors of the same plant. I know they come with pink, red and yellow flowers. Just an idea...

still leaning toward a Texas Mountain Laurel. I think the purple blooms would look good there. Might get a bit big for the space though.

It will be years before a Texas Mountain Laurel gets too big :-)

bamboo muhly is very pretty too.

you're right, but I don't think I've seen it in my nurseries. Too late anyway -- mountain laurel is in! Will post pic soon