The only trouble we initially ran into was that our original no-frills Freeview box wasn't listed on SlingPlayer's IR code database, and since the SlingBox couldn't be trained using our own remote control, we had to procure a compatible Freeview box.
The key is for them to play nice with each other, and the answer isn't a one-way interface like IR, or an HDMI pass-through with multiple user interfaces layered on top of each other.