I have yet to implement this, but in poking around in my car tonight diagnosing non-functional reverse lights (turns out the DRL add-on module somehow popped a fuse), I stumbled upon this little gem in one of the service manuals (BCS-8), regarding the combination switch / stalk in the car.
According to this diagram, once the tail lamp switch is closed, the BCM detects it by way of a signal passing from Output 4 to Input 5. Therefore, to ensure that you have tail lights and side markers at all time, you simply need to install a jumper between Output 4 and Input 5. You can either do this on the combination switch stalk, or on the plug at the BCM.
On Connector M24, that means a jumper between pin 33 and pin 2. Or, on the combination switch connector M27, pin 8 and pin 10.
I'm hoping to try this in the next day or two, and I'll report back on my success (or failure). If it works, I think this becomes the cheapest DRL mod, right?