Thanks for the recommendation. I hadn't actually looked at the weather before I posted this, and it's supposed to rain. I don't mind camping in the rain, but the point was to see Shasta, and that'll be hard if it's hidden in the clouds.
I think I've settled on the Sierra club Horse Camp from Bunny flat, a nice 2 mile hike.
But if the weather clears on Sunday, I'll definitely have to make a stop and Castle Lake and do the 2 miles round trip hike to Heart Lake for the view. If the weather ends up clearing on Saturday, Heart Lake might be the place anyways for that view.
No, I don't think you're ever supposed to camp within 100' (200') of a water source, but people are gonna do what they're gonna do.