How to redirect all Apache 2.4 websites to maintenance page while allowing access to specified IP addresses
Provided that your load-balancer forward the original IP one way or another (ProxyProtocol, X-Forwarded-For, ..) you could do something like that:
RewriteEngine On
RewriteCond %{REMOTE_ADDR} !^1\.2\.3\.4$ [NC]
RewriteCond %{REQUEST_URI} !maintenance.html [NC]
RewriteRule (.*) /maintenance.html [L,R=503]
Replace with your IP address and real maintenance page URL