Cool. I've had a look at how to do this, and what the problems might be.
Firstly you need to remove comments like this <!--- LINK TO YOUR TEMP PAGE FILE. They break the .htaccess file
Secondly, you need to keep in the original Rewrite stuff, to make sure that silverstripe will still process for you:
RewriteEngine On
RewriteCond %{REQUEST_URI} !(\.gif$)|(\.jpg$)|(\.png$)|(\.css$)|(\.js$)
RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* sapphire/main.php?url=%1&%{QUERY_STRING} [L]
So your .htaccess file should look something like this:
### SILVERSTRIPE START ###
<Files *.ss>
Order deny,allow
Deny from all
Allow from 127.0.0.1
</Files>
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteCond %{REQUEST_URI} !(\.gif$)|(\.jpg$)|(\.png$)|(\.css$)|(\.js$)
RewriteCond %{REQUEST_URI} ^(.*)$
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule .* sapphire/main.php?url=%1&%{QUERY_STRING} [L]
RewriteCond %{REQUEST_URI} !/maintenance.html$
RewriteCond %{REMOTE_HOST} !^888.888.888.88
RewriteRule $ /maintenance.html [R=302,L]
</IfModule>
### SILVERSTRIPE END ###
Hope this helps/works