If I'd like to draw an inner (inset) shadow, what are my options? Not supported by the framework, but can I do it by hand maybe?

Shadow modifier likely won't work so maybe with the previous Android Canvas? Or a border modifier with a gradient brush. @Nader Jawad might have better ideas.

Depending on the shape you could probably recreate it using a gradient and using

Modifier.drawBehind

to draw the inset shadow underneath the content

Nice, thanks for the tips, will play with gradients/drawBehind