I actually tried this with a Channel Mixer layer as a test with blue at 200%. Added this as an asset... doesn't have any thumbnail, so I pressed where the thumbnail should be, and selected "Insert". It definitely added the blue layer, but.it.does.absolutely.nothing!
Opening the Channel Mixer in Adjustments, I can see blue is at 200%. I can reset it, add different colors, etc. It does nothing. I have to delete that layer and add a new channel mixer layer, and then it will change the photo when I manually adjust colors.
When I purchased Affinity Photo, I was looking forward to be able to do some of this stuff on my iPad, but the limitations are astounding!