If you are on a NW (012) ticket you cannot Standby/FlyNow on a DL flight, unless the DL flight has a NW-codeshare flight number.

If you are on a DL (006) ticket, you can Standby/FlyNow (whatever DL calls it) onto NW flights only if it has a DL-codeshare flight number.'

Before too long, all NW flights will have a DL codeshare number. At that point it should be much easier to switch onto NW-metal on DL ticket stock. And at some point in 2010, there will be no more NW flights.

My guess is that it requires a who look of system changes to make some of this functionality work and it isn't worth the effort in the short-term.