So, assuming .htaccess in the document root, I think you need something like these:
RewriteCond %{THE_REQUEST} !^[A-Z]+\ /articles/index\.php\ HTTP/
RewriteRule ^articles/ /articles/index.php [L]
I am not completely sure you need the RewriteCond which will match any request
except GET /articles/index.php HTTP/
. The RewriteRule performs an internal rewrite of any path starting with articles/ (in this case, because .htaccess context, leave off the leading slash), serve the file /articles/index.php and stop processing more rules.
The bit I am still learning is that in the .htaccess context, any internal rewrite results in a recursive process of re-checking all existing .htaccess files using the newly rewritten path. I think the initial RewriteCond is needed to prevent the recursing iteration from matching again, in which case you would have an infinite loop.
I have completely ignored anything about dynamic URLs. I believe the problem, as I understand, is "anything in the /articles/ path should get the special index.php which returns a 503 response." If I have missed some nuance, then I probably need more info.
Let me know how this works, either way. I am well over 53% confident that I have this about right :-)
Tom