I just use this to load the RTF data. The article states '.txt' files, but RTF are just text files with funky formatting.
http://nsis.sourceforge.net/External_License_file
Also, if you want a thin border on the RichEdit box, just use the WS_EX_STATICEDGE flag instead of __NSD_Text_EXSTYLE.
Here's the full code I use:
PHP Code:
;License textbox (rich text)
nsDialogs::CreateControl /NOUNLOAD "RichEdit20A" ${WS_VISIBLE}|${WS_CHILD}|${WS_CLIPSIBLINGS}|${WS_TABSTOP}|${ES_READONLY}|${WS_VSCROLL}|${ES_MULTILINE}|${ES_WANTRETURN} ${WS_EX_STATICEDGE} 0 10u 100% 90u ''
Pop $txtLicense
;load the license from file
!insertmacro addLicense