I don't know - they were pretty adamant that The Emperor, Vader and The Jedi would not feature in the live action show but be background characters acting off screen. The show was to be about ground level characters and this doesn't sound like the plan for Rebels.
But look at how many different kinds of stories and subject matters The Clone Wars
tackled. If Rebels
is anywhere near as broad in focus, it could incorporate stories and ideas from the Underworld
series on occasion.