@temoto
To specify a 503 for bots and a maintenance page for humans, try the following
RewriteEngine on
RewriteBase /
#for bots such as google
RewriteCond %{HTTP_USER_AGENT} ^.*(Googlebot|Googlebot|Mediapartners|Adsbot|Feedfetcher)-?(Google|Image)? [NC]
RewriteCond %{ENV:REDIRECT_STATUS} !=503
RewriteRule !^/down/for/maintenance$ %{DOCUMENT_ROOT}down/for/maintenance [L,R=503]
#for humans
RewriteCond %{REMOTE_HOST} !^1\.1\.1\.1
RewriteCond %{REQUEST_URI} !^/maint.html [NC]
RewriteRule .* /maint.html [R=302,L]