Can I allow text messages from certain contact through when in Do Not Disturb mode?
Solution 1:
AFIK the Do Not Disturb does not have setting to let Text Messages through like phone calls.
You can join the large group asking Apple to do it http://www.apple.com/feedback/
Solution 2:
Edit the contact, and when you go to pick the text tone there is an option to bypass dnd.
This is done by enabling the Text Tone Emergency Bypass