mod_rewrite based on ip

You can use the REMOTE_ADDR variable in a RewriteCond

RewriteCond %{REMOTE_ADDR} !^10\.0\.1\.1$
RewriteRule ^ /maintenance.html

Just change the condition to match the IPs you want, for more than one you can use ^(ip1|ip2|...|ipn)$.

About how to disable the maintenance mode without changing the .htaccess file I think that's not possible short of writing a program that would delete it or otherwise modify it, an easy one would be to rename it.

I'd like to slightly correct Vinko Vrsalovic's answer.

RewriteCond %{REMOTE_ADDR} !^10\.0\.1\.1$
RewriteRule ^ /maintenance.html

This rule result will be infinite loop and HTTP server error, because it will be executed on redirection page too. To make it work you should exclude redirection page from the rule. It can be done this way:

RewriteCond %{REMOTE_ADDR} !^10\.0\.1\.1$
RewriteCond %{REQUEST_URI} !/maintenance.html$ [NC]
RewriteRule .* /maintenance.html [R=302,L]