The only thing that I can think of is to use a docking station. That way you can set the built-in agp to start, and the docking station video out as second. You may have to go into BIOS and set agp as first video.
I do this on servers and workstations, but I have never done it on a notebook. Let us know how it works. I use notebooks with bad screens as servers, since they are dying anyway, and I've had one going for 5 years now.
Now, back to BIOS. If you want kdenlive to use the external first, then go into BIOS and set the pci as first video. That way, even if you have a bad screen, you can still use the machine.