Yes, it is used for control. Any control packets are the highest priority, but after call setup, that 16k line sits idle. I know with PRI and T1 systems you can use the D channel (but it's 64k in those cases). I found an article that allows data over D-channel, but it says it has to use X.25:
http://www.eicon.com/support/helpweb/connt/PDX25D.htm
Personally, I don't think it sounds like it's worth it for the low speeds you'll get, but it might be useful if you are already running X.25 over IDSN (do they still do that?)