I ride up Piuma at least twice weekly. On weekday mornings (before 8am), via PCH Malibu Cyn is okay if you stay as far right as possible. Prevailing traffic is in the opposite direction weekdays early. Yes, you have to go through the tunnel, but best to block the lane and if you acknowledge any motor vehicles that you may be blocking, they are usually okay. Another route to Piuma involves more climbing, but that's what these ascents are all about. Early Sunday mornings turn on Topanga Canyon from PCH and climb up, left on fernwood-pacific, right on saddle-peak to top (great view of valley and ocean). Then you can either descend Stunt to Mulholland, left on Cold Cyn, left on Piuma...up, up, up. Or descend Schueren from saddle peak, right at the "T" which is Piuma...up, up, up to top of Piuma (another great view) and decend down to the bottom which becomes Malibu Cyn. I was actually out there this morning and it was about 102 degrees on Stunt.
Note: during weekdays from 7am to 3pm they are doing road construction on saddle peak and will not let you through to Schueren/Stunt. However, you can on weekends but be careful, there is loose dirt, gravel, and uneven terrain. Similarly, going up Las Flores is subject to 30 minute road construction delays at the point where the steepness starts, so trying to remount on a 14% grade is difficult if not impossible.