My suggestion would be to take the COMPUTE block out of your Proc Report. Your template specification of HSIDES is working as designed. The COMPUTE block LINE statement goes "inside" the boundary of the table. So the table is not finished until AFTER your LINE statement.
However, if you used ODS RTF TEXT= instead of the COMPUTE block, then you would get the the line under the last row of data in the table and your TEXT= string would be underneath the line.
This would mean that your style template would need to have the USERTEXT style element defined:[pre]
replace usertext from usertext/
but then your code would look like what's shown below. You might have to fiddle with the width and justification for the USERTEXT element, because it is now OUTSIDE of the table. In SAS 9.2, you'll be able to set the border top style separately from the other border styles, so you will use a different method in SAS 9.2 to change the interior table borders.
*** the code;
ods rtf file='c:\temp\test.rtf' style=newstyle;
proc report code; *<-- TAKE OUT COMPUTE BLOCK FOR LINE;
ods rtf text='This is table notes';
ods rtf close; [/pre]