A commented on Smashing Mag raised a good point: the mobile visibility classes could be a nightmare for screen readers. Going to investigate if there's a way we can alleviate that.
display: none and visibility: hidden will hide content from both browsers and screen readers, http://juicystudio.com/article/screen-readers-display-none.php.
If you still want to access your content from screen readers but hide it from browsers this is the way to go:
clip: rect(0 0 0 0);
Thanks kevva - that answers that for us. Of course at deploy the best practice would be to completely remove the hidden elements, but if not it sounds like we're still covered. Nifty.