Thanks, however, the meta tag won't work in this case because it's technically one page with an infinite amount of names via the URL (remember, the CMS only depends on the 123.html and ignores the directories preceding it). If I applied the NOINDEX within the meta, then the version I do want to get indexed would not be indexed.
The question was really around "will the internal rewrite of /123.html to just /123 be impacted if we disallow */123.html" - and since the rewrite happens before the bot sees it, I presume the answer is "no, it will not be impacted: 123.html will be blocked yet /123 will still be indexed.
Now, after I posted the question I realized this is the case where I should use a "greedy" 301 redirect via htaccess rather than try to block permutations of the URL via robots.txt. So I decided to not go the robots.txt route and instead do a 301 redirect via regex:
*/123.html to /123 (that's obviously not perfect regex, but you see my point)