To use a beamer, aticonfig can be a help:

1. Boot normally
2. Choose a screen resolution the beamer understands (in my case: 1280×800→1024×768). The laptop screen might look strange, but that's ok for now
3. Attach the VGA cable from the beamer
4. DISPLAY=:0 aticonfig –query-monitor (gives a list of attached devices, e.g. lvds and crt1)
5. DISPLAY=:0 aticonfig –enable-monitor=lvds,crt1 –effective=now (turns on beamer)
6. DISPLAY=:0 aticonfig –enable-monitor=lvds –effective=now (turns off beamer)

Allowed entities are: none,crt1,crt2,lvds,tv,tmds1,tmds2,auto

Note: Only two devices can be active at the same time.