After a few experiments, I landed on using #8 wood screws to mount the trim channel. Pan head screws were just tall enough to push the aluminum out a bit - though they are a better choice from an adjustability standpoint. With wood screws, you have to be more precise. The process I ended up with was:
1)
Level the trim channel
2)
At mounting point, use a countersink on level line (before drilling a pilot hole). This creates a relief in the wood, and a nice wood-screw friendly shoulder in the channel.
3)
Drill the pilot hole (countersink first does a better job of centering the hole in the center of the channel)
4) Drive screw