Assume my-blog-url is "http://myblog.com/"
I want to load a font-file from another server named 'http://somewhere-else.com/'

@font-face{
font-family:"Familyname";
src: url("http://somewhere-else.com/my-font.woff);
}

will be compiled to:

@font-face{
font-family:"Familyname";
src: url("http://myblog.com/http://somewhere-else.com/my-font.woff);
}

commenting line 251 in lib/Plugin.class.php (starts with add_filter('wp-less_stylesheet_save'…) did the job for me at first sight, but i don't see yet, what else might be broken.