I did some testing with my HP netbook and an external monitor, Slack 14 32 bit system.
I can turn on the external monitor with
xrandr --output VGA1 --mode 1024x768
I can then turn off the netbook display with
xrandr --output LVDS1 --off
. So far so good.
I shutdown the netbook through the gui, I run XFCE. I booted, with the external monitor connected. I see what you describe, the messages from the lilo boot screen to the first mode change appear only on the internal monitor. Once the modes set happens, ( font size changes ), I see the rest of the boot messages on both screens. The KDM screen shows up, I can log in. ( Still visible on both monitors), I log in, both monitors show the desktop. I can turn off the netbook monitor, either by the command line, or through the 'Display' app in the Settings menu.
What is a little strange, ( at least to me ) is the internal monitor is not in the correct mode. It normally runs in 1024x600 ( a 16x9 format ) and the external ( a 4x3 format ) is 1024x768. The bottom 1/3'rd of the screen is cut off on the internal monitor.
Now, if I try to turn the internal monitor back on, either with xrandr, or the Display app, I can not turn in on.
If I run shutdown, both work on reboot. I did get into a situation, once where I had both external and internal blank. I have not figured out how I did that though. I ssh'ed into the netbook, and ran shutdown.
So, in summary, it appears if both monitors are attached, on boot, the mode is picked for the external monitor. Don't know if you have another system to ssh to your system, if you do, that may be another way to gain control.
Not sure if the fact I can not turn the internal back on is a bug or not.
Hope this sheds a little light...