I've not tested this, but how about toggling the Excel Applications Visible property?

#Assuming your OLE Excel.Application object is called $Excel
$Excel->{Visible}=0;
$Excel->{Visible}=1;
[download]

Or how about toggling the WindowState property of the Excel object?

If it were the other way around it would be easy since Excel has an ActivateMSApp method. Unfortunately, WINWORD does not...