The only place that's reporting some crappie action is Indian Beach Resort in Clearlake Oaks. Their guests are catching 6 to 8 fish (2 1/2 - 3 1/2 pounds each) from their docks but 99% of the action is at night or early morning hours before daylight. I only mention this because it indicates a few crappie in the area. IBR is actually right on the Glenhaven/Clearlake Oaks border so most people consider it to be in Glenhaven.
I have heard of a few crappie in Rodman Slough but that's all I've heard. I imagine that if you worked the slough shoreline in the shaded areas like you do, you would catch a few but of course that's just my humble opinion.
The only thing I can add is that most crappie being caught this year are in the 2 to 3 1/2 lb class so it doesn't take a lot of fish to fill a basket.
As far as Highland Springs, I haven't even been there since late spring. At that time the boaters were catching a few bass but I didn't hear of any crappie there.
I wish I could give you more and better info but that's all I have. Perhaps some of our members can help out as well.
Good luck if you decide to come up.