I am new to the use of maps, but I suppose it would fit perfectly, using
core variables such as the binary IP address:
Maybe something like:

server {
error_page 503 /503.html # Configuring error page

map $binary_remote_addr $target { # Configuring white-listed IP
addresses
default KO
your_whitelisted_binary_IP_address_value OK
}

rewrite ^.*$ $target #Redirecting all traffic according to map-assigned
value

location @OK { # Named location to do nothing, i.e. serve content as
usual
}

location @KO { # Named location to trap maintenance traffic, spawning a
HTTP 503 error
return 503;
}
}

Untested, thus unsure, but I'd seek something looking like this.
---
*B. R.*
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