Não é perfeito porque os slides estão um pouco fora do centro, mas funciona bem o suficiente para mim. Basta executar o seguinte Applescript:
tell application "Keynote"
set theNumberOfSlides to count of slides in slideshow 1
end tell
repeat with i from 1 to theNumberOfSlides
tell application "Keynote"
activate
tell application "System Events" to keystroke "c" using {command down}
end tell
tell application "Pages"
activate
tell application "System Events" to keystroke "v" using {command down}
end tell
tell application "Keynote"
activate
tell application "System Events" to key code 125
end tell
end repeat