There's another code change that needs to be made; the mailer currently sends out HTML email encoded in ISO-8859-1, which is the default in libs/phpmailer/class.phpmailer.php. Rather than change that base library, I've changed libs/mailer.inc.php to set the character set explicitly to whatever is defined for display:-
PHP Code
--- mailer.inc.php.bak  2015-01-27 07:05:08.000000000 +0000
+++ mailer.inc.php	  2015-10-13 09:04:27.000000000 +0000
@@ -22,6 +22,7 @@
 
		$this->lang = $config['LANGUAGE'];
		$this->admin = $dotdot;
+		$this->CharSet = $ubbt_lang['CHARSET'];
 
		if (defined('IS_ADMIN')) $dotdot = "../";
 

This seems to correct the problem, and though Gmail doesn't handle the resulting UTF-8 properly (it corrupts the occasional character in long tracts of wide-character text, probably every 1200-1400 bytes or so on a line), both Thunderbird and Outlook seem fine with it.