Regarding removing the wet shine from areas...
Here's a trick where you can selectively remove bump and shine. Create a new layer. Set the layer blend mode to 'Multiply'. Set the layer bump blend mode to 'replace'.
Select the airbrush tool, and select the colour white (with no metallics).
You can now airbrush on your 'multiply' layer to selectively removed the bump and highlights from all the layers beneath, without affecting the colour at all.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"