I'm not sure if this is what you want, but...
http://www.cisco.com/en/US/docs/ios/12_4/secure/configuration/guide/h_sslvpn.html
Has an example of customizing your portal page.
eg:
webvpn enable
!
webvpn
logo file flash:/mylogo.gif
title-color #FF9933
text-color black
ssl encryption 3des-sha1 rc4-md5
ssl trustpoint WebVPN
url-list "quicklinks"
heading "Quicklinks"
url-text "Meetings and Conferences" url-value
"http://www.mydomain.com/resources/meetings.html"
url-text "Floor maps" url-value "http://www.mydomain.com/resources/floormaps.html"
url-text "Documentation" url-value "http://www.mydomain.com/eng/documents"
url-list "Departments"
url-text "Engineering" url-value "http://www.mydomain.com/eng"
url-text "Human Resources" url-value "http://www.mydomain.com/HR"
url-text "Sales and Marketing" url-value "http://www.mydomain.com/sandm"
url-text "Operations" url-value "http://www.mydomain.com/ops"
login-message "Enter your email-id and password"
!