is there a way to show an maintenance Page during "Restart Splunk", "SW Update",...
--> not the "not available page" rather an own service page
take a look at this app here https://github.com/M-u-S/SA-LoadBalancerStatus (offical Splunkbase app is not yet approved).
It provides a REST API endpoint that can be queried by the load balancer and based on the answer the instance will either be used or not.
This is not exactly what you asked for, but this way the users would at least not land on a instance that is about to be restarted or such.
Hope this helps ...
There isnt an official way. You would need to run an nginx proxy over the top and have that present the page.