Finally worked it out!
I put the landscape pages in a sub report at the end of the covering letter, preceded by a page break, and added the following code to the sub reports PageStart and PageEnd events:

Code:
private void SubReport_PageStart(object sender, System.EventArgs eArgs)
{
    this.ParentReport.Document.Printer.Landscape = true;
}

private void SubReport_PageEnd(object sender, System.EventArgs eArgs)
{
    this.ParentReport.Document.Printer.Landscape = false;
}