Is one able to "override" the terms of the Youtube Standard Licence with a slightly stricter Creative Commons than is allowed by the interface?
Don't do it

As @Mulvya mentioned, it is not a good idea to attempt to 'overwrite' the CC licence set in the video settings by putting a disclaimer in your video or video description. Apart from the legal implications and ambiguity, there's another problem: You're only changing the human-readable layer of the licence, while the machine-readable layer stays the same (an explanation on the three layers of the CC licences can be found here). What that means is that even though a human watching your video will be able to understand your intent to apply a more restrictive licence, a machine (e.g. an automated web crawler) will not. So your video will still show up in the search results when somebody searches for content licenced under the CC-BY licence. So by setting your video to CC-BY and disputing that claim in your video or video description, you are creating ambiguity for the user. I also doubt a disclaimer that the licence set by you in the YouTute interface does not apply would hold up in court, but IANAL.

Alternative

Hosting your video on YouTube with the intent to share it for others to adapt, remix and reuse is not a good idea in the first place. The stark compression applied by YouTube will significantly lower the quality of your video. The quality loss will only increase as others download your video, import it into their own projects, reencode it again and upload it to YouTube, where it's reencoded once more. On top of that, YouTube doesn't even offer a convenient way to download user-uploaded videos (though you can use them in YouTube's own video editor, but hardly anyone uses that anyway ...). For more information on YouTube's video compression, read my answers here and here. 

Instead, I suggest you upload a high-quality version of your video to a dedicated file hosting service and put your licence terms there. You can also bundle your video file with a text-file containing a description of your licence terms and a link to the online licence. This eliminates all ambiguity and allows everyone to use your video under clearly-defined terms. 

If you do this, put a link to the downloadable version of your video in your video description. Also, make sure to select the YouTube Standard Licence for this video. This allows users access for only private, non-commercial viewing purposes (see YouTube terms section 6). This way, you can make sure there's no ambiguity regarding your licence terms. As a sideeffect, this will point people towards the high-quality version, meaning nobody will use a low-quality version ripped from YouTube, in case you care about that.