That saddle wasn't tooled. At least not in the conventional sense, as in leather tooling, which involves cutting the grain side of the leather, with a blade, and using various shaped stamps to lower the leather in the desired pattern. But that must be done, usually, while the leather is damp, and before it is sealed or dyed, and finished.
The Brooks site says the saddle was done with an embossing plate. Which means the design was simply pressed into the leather. While slightly damp, I would guess. Then it was dyed and finished.

The problem with trying to replicate this is...your saddle has already been dyed and finished on the grain side. So you could try to add pigment to the already present embossing, but I fear it won't work.
You might be able to strip the finish off somehow, dye the embossing, and see if the darker dye stays when you wipe the surface. But I would think it would ruin the saddle.