Hi Gareth,
that syntax is used here: https://github.com/bmbrands/moodle-theme_elegance/blob/master/less/fontawesome/path.less being fa-version defined in https://github.com/bmbrands/moodle-theme_elegance/blob/master/less/fontawesome/variables.less#L9.
IMHO there is room for an improvement in Moodle for the work done in MDL-23493 BUT you know that Slash arguments set to On will be the default settings starting from 3.1 so I guess that this improvement will never land.
It could be coded directly into Elegance via theme_elegance_process_css() being declared as $THEME->csspostprocess (twice, a trivial bug): there it should be tested the Slash arguments setting and replaced accordingly the "?v="- I know you're not the maintainer just free discussion 
-, something like the patch below (untested):
diff --git a/lib.php b/lib.php
index ebfb973..ed6c05a 100644
--- a/lib.php
+++ b/lib.php
@@ -150,6 +150,16 @@ function theme_elegance_less_variables($theme) {
}
function theme_elegance_process_css($css, $theme) {
+ global $CFG;
+
+ // Fix the version used as a cache killer.
+ if (!$CFG->slasharguments) {
+ $css = str_replace(
+ array('?v=', '?#iefix'),
+ array('&v=', '&#iefix',
+ $css
+ );
+ }
// Set the background image for the logo.
HTH,
Matteo
