Suffice to say it's for performance reasons - you should only load CRITICAL assets in the head (preferably inline to reduce HTTP requests). All other assets should be loaded asynchronously or at the foot of the page.
So for now I've written a little function to do what I wanted, which I'll put below. It would still be good to know if the Requirements function can be modified though - optimising sites for mobile is hugely important, so being able to load CSS before the closing body tag is absolutely crucial.
class Page_Controller extends ContentController
* addUniqueQueryString() Adds a unique query string to a file name based on its modification date
* @param string $filePath Path to file relative to base URL
* @return string File name with unique query string appended, eg. `myfile.css?m=0123456789`
$fileLastModDate = filemtime(Director::baseFolder().'/'.$filePath);
$webSiteBaseURL = Director::BaseUrl();
return $webSiteBaseURL . $filePath . "?m=" . $fileLastModDate;
Then in Page.ss:
/* Site content... */
<link rel="stylesheet" href="$addUniqueQueryString('themes/mytheme/styles/myfile.css')">