You can do this with an applescript action!
The code is easy, something like:
tell application "Spotify" to playpause
The code I use is a bit more complicated. It won't open spotify randomly and it also stabilises the action so that if an error happens it'll be fine.
--control Spotify playback directly and not something else
if application "Spotify" is running then
tell application "Spotify"
try
playpause
on error
return "-"
end try
end tell
else
return "-"
end if
This code is from my preset AquaTouch, which has app-specific play/pause buttons for iTunes, Spotify and YouTube. Check it out!