Hi Dharmesh -
Tableau doesn't support users adding / displaying arbitrary web pages (in your case, a "welcome" page) into the Tableau portal. (Actually one can serve arbitrary content, but this won't help you).
Rather than driving yourself crazy trying to make this work, may I suggest another approach?
- Create a "welcome" page as a dashboard inside each of your workbooks that need one
- Instead of telling people to surf to http://tableauserver, have them surf to http://tableauserver/views/WorkbookName/WelcomeSheet
- User will be prompted to login, at which point they'll be forwarded to the "WelcomeSheet" dashboard inside the "WorkbookName" workbook
Hope this helps!