I want a self-destruct option on buildings.

He-he Your Emerald Spring killed you.
This will be quiet crazy ability. For same reasons you can't attack your own creatures. And it can't be described in normal way why you can self-destroy your buildings.

While we're at it, I'd like the ability to voluntarily "drop" an enchantment that you have cast...

The most simple way to exploit it is create domain giving building/enchantment, cast card that need it and destroy building/enchantment it to get your flux back.

Or perhaps we should have a new enchantment that gives "all figures you control gain: sacrifice to gain 1 flux". I like dynamics with an inherently self-destructive nature... they can offer great utility... I used to play with them in MTG all the time.

Not a bad thought. Nothing is worse than getting camped around a building that has 1 health and isn't next to a flux well. There's no recovery from that.

There is several 1st domain cards that can help you in such situation, but if enemu caught you this mean that you was doing something wrong.
Here's cards that can help: Mis-scribed Circle, Black Plague, Foul Termite Swarm, Fire Ball, Ascent, Hailstorm, Sunburst. Also in some cases Avalanche and Sand Storm can help you.