Ignore me, I just answered my own question. The switch works independently for each channel.

I just dialled in an input colour with red > 0.003131 and blue < 0.003131, and each channel uses the appropriate linear and non-linear part of the graph. So all we need is the Matmatic for this..... :O)

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)