I've done a lot of testing and have got all the rules I need working in a non-Wordpress environment but still using the default htaccess.
I have rewrite rules translating the last "folder" to a query string and I'm able to echo the $_GET['querystring']. This is just some 'querystring' for something else has nothing to do with WordPress.
/page/id5555/ to /page/?querystring=id5555
Although my theme is echoing this query string, WordPress is still interpreting it as looking for /page/id5555/.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^page/\b([A-z][A-z]\d{4})\b/?$ /page/?querystring=$2 [L]
</IfModule>
So at this point the new URL is /page/?querystring=id5555
Which next comes...
# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress
but WordPress tries to interpret '/page/id5555/' instead. Though I am able to echo $_GET['querystring'].