I know you can add custom HTML in the web.conf (login_content) but IMHO, SplunkWeb needs a restart. Can you add a custom message without restart?
post login, you can use messages to be displayed to users. I do this to inform users of outages, planned changes, etc.
Use something like:
curl -k -u adminuserid https://splunk.com:8089/services/messages \
-d name=message \
-d value="[$(date)] Notice Servers being rebooted at 3PM. Contact cmeinco with any questions or concerns."
Note: curl will prompt for the user password, instead of running with your pwd on the command line
Then after the change, i clear the message from the search head, since only admins can do this.
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You will have to restart, see the first paragraph in Customize the login screen in the documentation.
Any changes to .conf file directly would require a restart.