Sure it can be done.
You can use applescript to control Keynote or Powerpoint. So put another QLab session on your Slide presenting computer to receive OSC triggers for Applescript cue (that starts the slideshow or go to the next slide)
That's probably the easiest way but you can also use midi over ip and midi Pipe. Midi Pipe can receive a midi message then start an applescript. So you can put midi pipe on the powerpoint/keynote computer and send it midi messages. But now with OSC and QLab to QLab control it would be easier.
Here is a link for some applescript in Keynote: