Nice work Acolonna!
I noticed that if WordPress is not installed at the top of the domain, then WP Super Cache doesn’t seem to build the rules properly — it will set RewriteBase to the appropriate directory but doesn’t then use that folder in RewriteCond and RewriteRule, causing the matches to fail.
For example, if WP is installed into the /xx folder, the rules built will be:
RewriteBase /xx/
...
RewriteCond %{DOCUMENT_ROOT}/wp-content/cache/supercache/%{HTTP_HOST}/xx/$1/index.html.gz -f
RewriteRule ^(.*) /wp-content/cache/supercache/%{HTTP_HOST}/xx/$1/index.html.gz [L]
when it should be:
RewriteBase /xx/
...
RewriteCond %{DOCUMENT_ROOT}/xx/wp-content/cache/supercache/%{HTTP_HOST}/xx/$1/index.html.gz -f
RewriteRule ^(.*) /xx/wp-content/cache/supercache/%{HTTP_HOST}/xx/$1/index.html.gz [L]
I like your modification of the RewriteRule too; I noticed I was seeing an extra trailing backslash (in the RewriteLog) like:
path//index.html.gz
Another way to fix this up is:
RewriteRule ^(.*?)/*$ /wp-content/cache/supercache/%{HTTP_HOST}/$1/index.html [L]