Try to create the page with custom message about the load. when user try to login during the load run set the portalpath to that page. So on logon user can see that message.
if helps mark
If you want to do this at a dashboard level, You will need to create a simple new report and use it in your conditions on the dashboard sections. So this is how it goes.
If you have a set time frame for the loads and set time frame to show this message.
Then you can create a new analysis that will just display the "Current Time" and if the current time is between your data load window then you will get one row, or else no rows.
Then use this analysis as a condition that will not let any of users to access any sections of your dashboard when a row returns for that analysis.
This way you can control the behavior of users using the dashboards. You can also try and display a custom message when the data load is in progress.