I found a way to do it, but I need to perfect it yet. For anyone interested in some trials (I'm not posting a tgd, that would be too easy); use a ( blue node ) get position in texture, a perlin 3D and a constant scalar for scale, then a smooth step, then a color adjust unclamped. From the smooth step displace up by displacement shader, followed by a compute normal (very small patch), fed into child of surface shader, which is masked by the color adjust (which needs some tightening of WP and BP), give the surface shader some (rather much) offset laterally. Then feed that surface shader into the main input of another surface shader, masked by the same color adjust, and have the initial displacement shader fed into its child. Then give the second surface shader some negative vertical offset.
Now I want to try have some stones 'oversnowed', and more control over placement.
These stones are only on the snow here (displaced in test 2), as that's where I sat them on. So on dry land I made a different set, consisting of 'ordinary' fake stones.