Gotta love calling back. So here's what happened to me:

I purchased a Z fare reservation one way BOS-LAX (via IAH) for $530. The latest departure I could get was the 4:20 pm, but I really wanted more time in BOS.

I called CO to ask about the later flights. While I saw that all were sold out, I asked if my ticket could be used toward DL or NW.

Agents 1 through 5 said "no." They all added that only the codeshare flights would work.

So I did some reseach, and I found that I could take DL flights with CO numbers via CVG departing at about 5:30.

So I call back CO. Agent six. Though I told the agent the CO flights numbers for the codeshare, she could not find them. So she simply booked me on the DL flights with the DL flights numbers. She re-issued the tickets, and I seem to be all set.

6th time's a charm!

But it leads to a question. As I was on an unresticted ticket, shouldn't I be able to use it toward any carrier anyway?