Hi Dave, I'm glad you are interested in generating 405 video this way.
I don't know about using the Y/C output, I didn't have any luck when I gave it a brief try using an ATI PCIe card.
Does the following modline show up as valid?
Modeline "664x377_25 10.1kHz 50.0Hz" 8.100 664 680 752 800 377 378 385 405 interlace -hsync -vsync
If so, does your laptop have a VGA socket that can be connected to a monitor and be configured to "extend" the screen of the laptop?
Butcher a VGA cable like the one in the diagram in the link below, to matrix the R,G, & B plus the H&V syncs into a composite monochrome waveform, and activate the modeline using the secondary display (Video 1:...) no other configuration should be necessary.
http://www.nostalgiatech.co.uk/240%20lines.htm