Extra params in URL for custom page cause not found

I am using WP 1.5, without mod_rewrite, and my permalink template is this: /index.php/%post_id%/

I expect heavy discussion on a new blog and I’ve decided to show comments on a separate page. I’ve created a page template called discussion.php and I’ve assigned it the slug “discussion”.

I call that page using URL’s such as this: /index.php/discussion/10/, where 10 is the post id. After doing some additional tricks in the template, I replace the page $post with the requested post (10) and I proceed to display the comments. All well so far.

Next, I thought I’d make the same page also serve a separate content, with only the comment posting form (why? because I’m also going to have threaded comments and I don’t want to use JavaScript for “reply to this comment”).

So I tried using some of the following URL’s, only to be greeted by WP’s 404 template page:
/index.php/discussion/10/reply/
/index.php/discussion/10/reply/quote/
/index.php/discussion/q10/

I’ve confirmed that discussion.php is never even reached when using the above URL’s. Why is this happening and what can I do to make them reach discussion.php?

The workaround would be using /index.php/reply/10/ and /index.php/reply/010/ (for simple reply and with-quote, respectively). But I don’t like it much.