Yes, the corner HTML can be set using BeforeHeaderRender, check for e.IsCorner.
Example from Demo/Calendar/Default.aspx.cs:
protected void DayPilotCalendar1_BeforeHeaderRender(object sender, BeforeHeaderRenderEventArgs e)
{
if (e.IsCorner)
{
e.InnerHTML = String.Format("<div style='padding:2px; text-align:center; font-weight: bold;'>{0}</div>", DayPilotCalendar1.StartDate.Year);
}
}