Twilio evangelist here.
Unfortunately there isn't any way to transparently include metadata in an SMS and have that metadata returned to you when they reply, but there ways to address your scenario.
If you are sending the user a text message you already have their phone number and you can use that as a unique ID. When the user replies to your message, Twilio will as part of our HTTP request to your application, pass you the users phone number. You can use that number to locate them in your database and mark them as confirming or canceling.
Hope that helps.