Which direction of a forward slip is safer during a turn to final? Suppose I'm making a turn from a left base to final and, realizing I need to kill quite a bit of excess altitude, decide to use a forward slip with full rudder deflection (along with full flaps).

Which direction of rudder deflection would be preferable?

I suppose that yawing left would result in less bank and therefore a lower load factor, which may be safe at low speed and low altitude. But yawing right, will result in a steeper bank which (under the same IAS) will result in an even higher rate of descent thereby getting me to the ideal glide slope earlier and leaving more time to fully stabilize before the threshold.
I use slipping turns all the time.  

If you want to do a slipping turn you yaw the nose OUT of the turn, being careful to keep G loads low. It's NOT a dangerous maneuver if you keep speed up and nose down while doing it although you should use a bit more aggressive nose down than in a straight side slip to account for the slight extra G from the turn.  If you keep the wings largely unloaded and let the airplane fall semi-ballistically, you aren't going to stall.

To do it safely however, you do have to know how to fly the plane.