<s>You could try adding the opposite of your current speed vector to the clouds scrolling speed.</s>

After experimenting I've come to the same conclusion as you. :(

My testing involved teleporting the camera to coordinates and changing the shapeoffset to match. Since the shapeoffset isn't in metres, I found the unit conversion by moving the camera and offset at different rates*deltatime on the same axis until they both appeared to be standing still.

1m world space = ~0.00005f in cloud offset space. 
 
The main 'cloud shapes' stay in the same relative positions, but the rest of the noise doesn't seem to be accessible.
 
 
I found this post from a dev saying that shape offset should be enough: https://forum.unity.com/threads/volumetric-clouds.1078811/page-10#post-7600129 - so maybe some recent features have made this not possible to do.

The only ideas I have left are to try putting your own Cloud Map and Lut under Manual Cloud Control, and then see if the ShapeOffset and/or CloudMapOffset get nice results. 
Could also try scrolling the texture UV's. (I wish I could test it but couldn't find out how to make those textures, or find pre-made ones)