ac-13

In the forum Feedback for Affinity Photo on Desktop, this post - 
- talks about the way that cut/paste works in macros. You could add your vote in support by posting there, with a "+1"

I downloaded your image and did the following:
Convert to Greyscale
Add a Curves Adjustment layer - see screen shot- drag the left-hand bottom (Black) node about halfway up the left side
Set the Blend Mode of the Curves Adjustment to Negation
Use the Blend Ranges option on the Curves Adjustment layer - the cogwheel shown on the layers Panel beside the Blend Mode - and set these as per the second screen shot, with the Underlying Composition Ranges dragged down to zero (bottom) about 50% of the way along
Trousers are greyer than original, maybe not as light as you want though...

One way to bring in a Channel from another image is to use Apply Image filter.
The terminology is that the image which you want to edit, the image into which you want to place a channel taken from another image, is the Destination D. The image from which this channel is taken is the Source S.
Open Destination image. Then  Filters -> Apply Image.
Click on Load Image, and a file selector window will open. Locate your Source image file, then Open.
Ensure that Scale Horizontally to Fit and Scale Vertically to Fit are ticked (should be by default).
Tick Equations, and the next section of the dialog will appear instead of being greyed out.
Ensure Equation Color Space is RGB.
The 4 equation lines below are for DR, DG, DB and DA. These are the D(estination) R(ed), B(lue), G(reen) and A(lpha) respectively.
Set DR = DR and DB = DB  and DA = DA - keeping the Red and Blue and Alpha Channels on the Destination image equal what they were before.
Set DG = SR
This sets the Green Channel on the Destination image to equal the Green Channel from the Source image.
The James Ritson tutorial on Apply Image - Equations shows this feature in action.

When this thread and the tutorial with the solution were posted by  @dmstraker, back in December, I read them with interest. Around that period there were also some posts about the use of Equations in AP. As H, S and L can be derived from R, G, B as formula calculations, I thought it could be possible to build a saturation mask in this way, with Apply Image.
Details of the calculations were from Rapidtables RGB to HSL Converter
The calculations are as follows:
R,G,B = Red,Green,Blue as values in range 0 -> 1 - so divide ordinary R,G,B by 255
L(ightness) = ((Max(R,G,B)+Min(R,G,B))/2)
Delta = (Max(R,G,B)-Min(R,G,B))
S(aturation) = Delta / (1-Abs((2*L)-1))
 
Having selected the layer required, Filters -> Apply Image ->Use Current Layer as Source then tick Equations, and ensure Color Space is RGB.
In terms of SR, SG and SB,
S(aturation) = (Max(SR,SG,SB)-Min(SR,SG,SB))/(1-Abs((2*((Max(SR,SG,SB)+Min(SR,SG,SB))/2))-1))
and so set Destination R, G and B all to that value
DR, DG and DB = (Max(SR,SG,SB)-Min(SR,SG,SB))/(1-Abs((2*((Max(SR,SG,SB)+Min(SR,SG,SB))/2))-1))