I definately think that helping your girlfriend to see the link between you being locked up and her life becoming much more pleasant is the key here.
If she has ever wondered where the initial passion went, and all the dating and sexy texts and phonecalls that happened in the early relationship days disappeared to, that is a great time to bring up chastity.
For example, if she enters into the "you've changed" speech (we all do it from time to time!) then why not apologise and say you realise you take her for granted these days and it was different when sexual activity was restricted and you had to work hard to keep her happy and hopefully she would be receptive sexually, whereas these days you can roll over in bed, give her a kiss and hope for the best!
This can then lead you into ideas on how to regain that sexual restriction and get you to work harder to be rewarded!
As you have already dabbled in chastity, then show her when you are locked up that you are much more caring and devoted, perhaps be a bit of a brat after you get sexual release! Devious, I know, but you may find yourself locked up longer next time!
Hope that helps on opening up the subject.