Juncus mertensianus looks similar except it doesn't occur in AR?

Thanks saltcedar. That gives me a place to start. I'll wait for the plants to mature, and try to key one of the stems out. It does look unlikely that J. mertensianus would be found in this area, but we do occasionally see strange things come in with building materials.

The way the bract appears to be a continuation of the culm suggests the subgenus Genuini, however your description of the leaves makes it sound like they are septate. If you run your thumb and index finger down one does it feel bumpy at regular intervals?

Yes, definitely septate. I cut one open to confirm. One of my more obvious failings is that I try to ID plants before they are mature. That really gets me into trouble with the sedges and grasses :).