@dmke, I saw a recent thread in this forum, not quite same problem, but thought of your SMS 
issues. So apparently someone did write PDU and i18n encoding parsing logic in RouterOS script, quite the doozy: http://forum.mikrotik.com/t/script-for-sending-incoming-sms-to-mail-with-full-parsing/140125/1 
– you case is actually the reverse, but perhaps one of the encoding helpers there could be used to generate the right sequence for “at-chat”, somehow – but I don’t know Go, that still seems easier