OK, I can give good information here.

In two words: Forget it.

While in theory CO BusinessFirst upgrades would be available with KL miles, I have tried for four different itineraries, involving 15 or so agents, and have come to the following conclusion:

CO doesn't really want to have Upgraders in their premium cabin. Maybe, if they paid A LOT OF MONEY for a high Y fare, and can be sure that BF will be more than half empty, they will consider it. This conclusion can be drawn by looking at the CO board, and is based on my own CO experience (was elite several years there).
This is why CO has constructed an intransparent procedure for the FD Helpdesk, which allows them to control (this means limit) the amount of upgrades they give to KL members. The procedure is that the FD agent has to send requests for upgrade seats to Continental. CO has a desk (I assume live people) that takes minimum 1 to 2 weeks to evaluate the request. This desk then sends a final answer back to CO.

On two upgrade requests that I put in, CO BF award availibility for CO members was there. The other two requests had over 9 seats in J and D all the way to departure date, but no award seats.

Not one of my requests cleared. They all were refuted between one week and 70 hours prior to departure. Even when I requested 6 weeks before departure time. Note that CO only allows (or has this changed recently?) CO members to waitlist between 30 days prior and 72 hours prior.

I even asked one of the agents wether any request he witnessed had ever cleared. He admitted that out of 50-100 requests he had so far only one request had been confirmed. I asked him for the fare class - he said he couldn't tell me - I asked if it was full Y - he said yes.

My final conclusion: The folks at the CO partner desk won't give us upgrades, and for sure not on low fares. MAYBE on the rare occasion that an almost entirely empty BF cabin is expected with certainty they will give folks on a K-fare (or equivalent) and up an upgrade. But, IMOP, they won't give any upgrades except you paid almost as much as you would have had to pay for BF anyway and aren't expecting a full cabin.

KL FD staff can't do anything. They have one supervisor who is allowed to directly correspond with CO. And this person also views CO reply's as final. FD helpdesk people are poor folks because their management gives them so limited, stupid and bureaucratic procedures that their job mission of good customer service becomes really hard to fulfill.

Stick with KL metal and you'll be happy. Stick with KL and NW metal and you'll be mostly happy.