How can coefficient of lift on the ground with flaps in the takeoff position be calculated using airfoil data?
There is quite a lot written on the topic, especially as part of "ground effect" vehicles research.
For example, Cui and Zhang cite two different types of lift coefficient. The first one (simpler and given here) is based on flat plate airfoils and is a modification to the well-known
$\frac{dC_l}{d\alpha}=2\pi$:

$$C_L=2\pi\alpha(1+\delta^2)(1-2\zeta)$$

with

$$\zeta=\frac{\sin\alpha}{4(h/c)}$$

and 

$$\delta=\frac{\cos\alpha}{4(h/c)}$$

In the above expressions, $h$ is the height of the wing above the surface (ground, water, etc.), $c$ is the chord and $\alpha$ is the angle of attack.

I strongly recommend you check out "ground effect"/"Ekranoplan" literature - should be useful.