The requested feature is already partially implemented in a different way....
KeyboardWarrier's Tip of the Day 4 thread states:
First Layer Settings (These are applied to where the Bed touches your model. If you have a raft, that means the First Layer Settings will apply to your raft).
First Layer Height: This % will take a % of your Primary Layer Height. If you are printing with a small Primary Layer Height like .1 mm I would recommend a First Layer Height of 250%, to get about a resulting .25 mm first layer height
If your First Layer Height is below 100%, the extrusion amount will remain the same, only the Z-will change, but if you increase above 100%, the extrusion amount will scale accordingly.
First Layer Width: the extrusion width of your first layer, you may find that your first layer sticks better with a thicker extrusion (100%+). I don't have too much experience with this, but I think 125% or 150% would be good starting points.
First Layer Speed: Slows down the First Layer Speed to a percentage of your Default Printing Speed (Other tab)
So.....to increase the amount of plastic being extruded relative to the layer height on the first layer....ie increase the extrusion multiplier....simply reduce the layer thickness below 100%.
Personally, I adjust layer thickness and global-z position in combination to achieve exactly the same result....which I'm perfectly happy with. I would prefer to see development time being used to make changes that can't be done any other way (other than using a different slicer) - like sorting out thin wall behaviour!