As you've seen keywords are not included in the metadata that can be added to a slide.
To accomplish this you'd have to copy the keyword(s) into one of the available metadata fields such as caption. Unfortunately there is no automatic way to do this in Aperture. It would be possible to do it through a Applescript if you have any scripting experience. The code wouldn't be that difficult to write.
Currently Being ModeratedJan 22, 2013 2:01 AM (in response to Frank Caggiano)
Is this still the case nearly one year later (referring to the recent discussion on my question "From Aperture to iPhoto", with screenshots, on the iPhoto support community forum, answered by Terence Devlin)?
Currently Being ModeratedJan 22, 2013 10:06 AM (in response to Frank Caggiano)
As another work-around, that I sometimes use, set the free BorderFX plug-in (BorderFX - BorderFX) to create an annotated version of the image. You can write the keywords directly onto the image or onto a custom border of the image. This is highly customizable. Just add a textbox and set it to show the IPTC field "Keywords".
Then use the annotated versions of your image in the slideshow.
For example: The keywords written onto the border below the image.