Combining the ideas of the shammy gathering sweat, and the shammy drying rapidly in sunshine and breeze, what about the idea of a shammy extension that would wick the sweat further out, and maybe around the side of the pack where there is better evaporation from the breeze and maybe some sunshine.

Use the shammy as a transporter of sweat from where you don't want it to where it can evaporate. You'd need a bigger shammy - and that means more weight. And it might not look so nice unless you were creative in shaping it to the pack.

Unfortunately, it might also be impractical.