I just had this problem. I just bought a DCA-MSP-ORD-MSP-DCA flight on Travelocity. All segments obviously operated by NW, but it's a CO ticket and all segments are CO-designated.

I credit to NW so I don't care about the online bonus, and I'll get 100% EQMs and almost-guaranteed upgrades since it's NW-op (showing off!) but I would have liked the 24 hour hold.

Yet even when I tried to force it by booking it as multiple segments, it gave me only CO-operated flights (DCA-CLE-MSP) etc... When I requested nonstop only, it told me CO doesn't offer service.

So I bought on NW. Any way around this?