Posting here as a fallback I've implemented as I was depending on the ChosenJS plugin to work so that custom CSS could be applied. Hopefully this helps someone else.
Disclaimer: The answer above by @dreamweiver should still be the accepted answer, given the question.
var chosenSelects = $('.ui-select').find('.chosen-select, [chosen]'),
$select, $option;
if (chosenSelects) {
chosenSelects.chosen();
// Check for 'chosen' elements on mobile devices
// -----
// Given that ChosenJS has expressly been disabled from running
// on mobile browsers, the styles have to be applied manually.
// Source: https://github.com/harvesthq/chosen/pull/1388
// -----
// The code below gathers all 'chosen' selectors and adds
// 'chosen-mobile' as a className. This className is then
// used to apply the necessary styles for mobile browsers.
// Within each select, if an 'option' has an empty value,
// then that value will be given 'selected' and 'disabled'
// attributes to act as a placeholder, adopting the text
// in the 'data-placeholder' as the text to be displayed.
if ( /iP(od|hone)/i.test(window.navigator.userAgent)
|| (/Android/i.test(window.navigator.userAgent) && /Mobile/i.test(window.navigator.userAgent)) ) {
chosenSelects.each(function () {
$select = $(this);
$select.addClass('chosen-mobile');
$select.find('option').each(function () {
$option = $(this);
if ( $option.val() == '' ) {
$option
.attr('selected', 'selected')
.attr('disabled', 'disabled')
.text( $select.data('placeholder') );
}
});
});
}
}
With this, I then use .ui-select .chosen-mobile
to apply the CSS necessary.