If you don't want to go with the 'write your own shader' option, what you're trying to do is quite possible.

Probably the most straightforward way to manage this should be this:

var newValue = GUILayout.HorizontalSlider (charCam.GetComponent(ColorCorrectionCurves).redChannel[1].value, 0, 255);
charCam.GetComponent(ColorCorrectionCurves).redChannel.MoveKey(1, new Keyframe(1, newValue));

(to a given definition of 'straightforward'...)

EDIT nah screw that, here have a 'brightness' screen effect.

CODE:

using UnityEngine;
using System.Collections;

[ExecuteInEditMode]
public class ScreenBrightness : MonoBehaviour
{
    public Shader   shader;
    public float brightness;
    private Material m_Material;
    // Called by the camera to apply the image effect

    protected Material material {
        get {
            if (m_Material == null) {
                m_Material = new Material (shader);
                m_Material.hideFlags = HideFlags.HideAndDontSave;
            }
            return m_Material;
        } 
    }

    protected void OnDisable() {
        if( m_Material ) {
            DestroyImmediate( m_Material );
        }
    }
    void OnRenderImage (RenderTexture source, RenderTexture destination) {      
        material.SetFloat ("_Intensity", brightness);
        Graphics.Blit(source, destination, material);
    }   
}

SHADER: You need to attach this to it before it'll work.

Shader "Hidden/Brightness" {
    Properties {
        _MainTex ("Base (RGB)", 2D) = "" {}
        _Intensity ("Brightness", Range (0, 1)) = 1
    }
    // Shader code pasted into all further CGPROGRAM blocks
    CGINCLUDE

    #pragma fragmentoption ARB_precision_hint_fastest
    #include "UnityCG.cginc"

    struct v2f {
        float4 pos : POSITION;
        half2 uv : TEXCOORD0;
    };

    sampler2D _MainTex;
    float _Intensity;
    v2f vert( appdata_img v ) 
    {
        v2f o;
        o.pos = mul(UNITY_MATRIX_MVP, v.vertex);
        o.uv = v.texcoord.xy;
        return o;
    } 
    fixed4 frag(v2f i) : COLOR 
    {
        fixed4 color = tex2D(_MainTex, i.uv); 

        return color * _Intensity;
    }

    ENDCG 
Subshader {
 Pass {
      ZTest Always Cull Off ZWrite Off
      Fog { Mode off }      

      CGPROGRAM
      #pragma vertex vert
      #pragma fragment frag
      ENDCG
  }
}
Fallback off

} // shader