Access Master Page Controls from Content Page?

Yes.  We can access master page controls from the content pages.  Below are the  examples towards the same.

Example1:

TextBox tb = (TextBox)Master.FindControl("txtMaster");
tb.Text = "Something";

Example2:

ContentPlaceHolder mpContentPlaceHolder;
TextBox mpTextBox;
mpContentPlaceHolder = (ContentPlaceHolder)Master.FindControl("ContentPlaceHolder1");

if (mpContentPlaceHolder != null)
{
    mpTextBox = (TextBox)mpContentPlaceHolder.FindControl("TextBox1");
    if (mpTextBox != null)
    {
        mpTextBox.Text = "TextBox found!";
    }
}

Example 3:

// Gets a reference to a Label control that is not in a
// ContentPlaceHolder control
Label mpLabel = (Label)Master.FindControl("masterPageLabel");
if (mpLabel != null)
{
    Label1.Text = "Master page label = " + mpLabel.Text;
}