Taken on a small group of Islands in the Oslo fjord, called Hvasser. A 15 meter peice of fabric playing in the wind, scanned right to left in 21 seconds. Got really lucky with the clouds this time, allowing a single beam of sunlight in as a highlight.
What you are describing is known as a harris shutter, which would be awesome to explore, especially considering the weird way motion is captured on one of these devices.
I don’t think a separate brightness channel is entirely necessary. But if you want to go down that path, I’d start trying out the blending modes in photoshop. There’s probably also some method for taking your RGB composite, and bring it into a colorspace that has a separate channel for brightness, and then replace that channel with your b/w pass. Lab color space should allow you to do this, although I’m not really sure what software to use, or if theres some way of doing This directly in Photoshop.