I fools the BIOS into thinking that a VGA monitor is attached. When Slacko has started, it asks for a monitor. With a VGA monitor, data is read from the monitor EPROM, but now there's no data. I choose LCD display 1024x768x24 which is the maximum what my ATI Radeon can do. Reboot, save slackosave and let it restart. Voila!
I use the first hardware solution. Slacko runs on a headless laptop with X11VNC and my MacMini runs TigerVNC over wireless. Flawless!
you cannot see the boot sequence in this way. however, my laptop
has a s-video output which displays the boot sequence on an external monitor with s-video in. s-video switches off when boot is complete