mod_rewrite with WP

I’ve extended WP by making a custom gallery system as a page template, which uses lightbox and supports linking to (by passing parameters through the URL). I want to rewrite the URL so that it is pretty, basically, but it seems that WP interferes.

I basically want to turn
from ?page_id=ID&imgcat=XXX&imgname=YYY
to /pagename/XXX/YYY

Clearly there’s no way to turn the page_id into the page-name, so I’m happy to have a seperate rewrite rule for each such page.

My problem is that if you have anything other than a number after /pagename/ WP returns a 404. How do I get around this?

This is the contents of my .htaccess file, which is in my root directory:

Options +FollowSymlinks
Options -Indexes

RewriteEngine On
RewriteBase /
RewriteRule ^fable-2-gallery/([^.]+)/([^.]+)$ index.php?page_id=14&imgcat=$1&imgname=$2 [L]

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
# END WordPress

Help making this work would be much appreciated.