Below I’ve posted something I mocked up going for deformable pixel terrain. You may be able to get something from it… You may not… Either way, It can obtain the pixels of an image around the mouse position. The names are pretty self explanatory but if you don’t understand something, post a comment here.

using UnityEngine;
using System.Collections;
using System.Collections.Generic;

[RequireComponent (typeof (SpriteRenderer))]
public class BitmapMask : MonoBehaviour 
{
    public Texture2D spriteTexture;
    public Texture2D maskedTexture;
    public int diameter = 50;

    public List <bool> mask;

    private float _width;
    private float _height;
    private Sprite _image;
    private SpriteRenderer _spriteRenderer;

    private Color[] tempColors;
    private Color[] maskColors;

    void Start () 
    {
        _spriteRenderer = GetComponent <SpriteRenderer> ();
        maskColors = maskedTexture.GetPixels ();

        _image = GetMaskedSprite (spriteTexture, maskedTexture);

        _spriteRenderer.sprite = _image;

        _width = _image.bounds.size.x * _image.pixelsPerUnit;
        _height = _image.bounds.size.y * _image.pixelsPerUnit;
    }

    void Update () 
    {
        Vector2 mousePos = Camera.main.ScreenToWorldPoint (Input.mousePosition);

        mousePos += (Vector2)_spriteRenderer.sprite.bounds.extents;
        mousePos *= _spriteRenderer.sprite.pixelsPerUnit;

        if (Input.GetMouseButton (0))
        {
            ChangePixels ((int)mousePos.x, (int)mousePos.y, diameter, Color.black);
        }

        if (Input.GetMouseButtonDown (1))
        {
            ChangePixels ((int)mousePos.x, (int)mousePos.y, 50, Color.white);

        }
    }


    Sprite GetMaskedSprite (Texture2D spriteTexture, Texture2D maskTexture)
    {
        Texture2D texture = new Texture2D (spriteTexture.width, spriteTexture.height);
        Color[] spriteColors = spriteTexture.GetPixels ();

        tempColors = new Color[spriteTexture.width * spriteTexture.height];

        for (int y = 0; y < maskTexture.height; y++)
        {
            for (int x = 0; x < maskTexture.width; x++)
            {
                bool masked = (maskColors[x + y * maskTexture.width]) == Color.white; // change to white and change actual image

                if (masked)
                {
                    tempColors[x + y * maskTexture.width] = new Color (0, 0, 0, 0);
                }
                else
                {
                    tempColors[x + y * maskTexture.width] = spriteColors[x + y * maskTexture.width];
                }
            }
        }
        texture.SetPixels (tempColors);
        texture.Apply ();

        return Sprite.Create (texture, new Rect (0, 0, texture.width, texture.height), new Vector2 (0.5f, 0.5f));
    }

    void ChangePixels (int centreX, int centreY, int diameter, Color color)
    {
        int radius = diameter / 2;
        int a;
        int b;

        for (int y = centreY - (diameter/2); y < centreY + (diameter/2); y++)
        {
            for (int x = centreX - (diameter/2); x < centreX + (diameter/2); x++)
            {
                if (x < 0 || y < 0 || x >= _width || y >= _height) continue;

                a = x - centreX;
                b = y - centreY;

                if ((a * a) + (b * b) <= (radius * radius))
                    maskColors[x + y * maskedTexture.width] = color;
            }
        }

        ChangeSprite (GetMaskedSprite (spriteTexture, maskedTexture));
    }

    void ChangeSprite (Sprite updatedSprite)
    {
        _spriteRenderer.sprite = updatedSprite;
    }

}

Basically it takes in a spriteTexture and a maskedTexture. The masked texture gets altered to either black or white around the mouse position when a click is detected. The masked texture is then used to determine whether pixels from the sprite Texture should be shown, obviously based on the colour.

It may not exactly be what you’re looking for but its a start in the right direction. Oh yeh and its not optimized and I have no idea about the bugs…

Goodluck.