Thanks, LaurieA and ima70, for showing that a sitting-down pose *is* possible!

I've given up on my own attempts at such a pose for now and gone back to a simpler pose.

The attached picture shows a dynamic skirt simulated with the following settings...

Frames: 60
Steps per frame: 15
Object vertex against cloth polygon: Off
Object polygon against cloth polygon: On
Cloth self-collision: On

Collision Offset: 0.3
Collision Depth: 0.3
Static Friction: 0.5
Dynamic Friction: 0.1

Collision Friction: No
Fold Resistance: 40
Shear Resistance: 150
Stretch Resistance: 100
Stretch Damping: 0.1
Cloth Density: 0.1
Cloth Self-friction: 0.5
Static Friction: 0.8
Dynamic Friction: 0.6
Air Damping: 0.02

Groups
Constrained: Waistband vertices
Default: Everything else

The skirt was set to collide with the figure's hip, abdomen, buttocks, and thighs, and also with the hip and abdomen of the vest.

A few small dots of poke-through and some strange wiggly effects on the waistband. From this angle, it looks like the skirt waistband might be sinking into the vest but it's not... the wiggly effect seems to be the vertices of the waistband jumping around a bit, several millimetres away from touching the vest.

Does anyone have any suggestions about how to change the settings and/or do something else to remove these problems? I mean, actual numbers and techniques to try, since "Try lowering value X" could mean I spend the next week trying every possible lower value for X! If anyone offers some decent-sounding suggestions, I'll give them a go and post the results.