I love the idea of spelling it out in nautical alphabet flags. Keeping the entire quilt truly nautical. You would have to do a little research. Although you would have to give her a flag alphabet so she could decipher it - that would be fun to video tape her "translating it". Do you live by the beach? I also like the idea of putting the ring box in an envelope attached to the back. What you could also do is, inside the envelope attach the ring to a ribbon that is very securely stitched to the pocket itself (thus eliminating the bulk of the box). Definitely go to your local quilt shop - the quilters there will be happy to help you. They would be overjoyed to assist a young man bring his wonderful plan to life. I wish you much love and much luck. You can do this - but start right away so you don't feel rushed. Good luck with this and your future endeavors. You're very creative and thoughtful! God Bless!
PS you could get lots of help from the family as long as they don't know what these blocks are. Just ask them to help you make the blocks you need. Don't show them the entire alphabet - just pull out the ones you need. Use similar colors so they match the rest of the background fabric you choose. This could be a truly awesome quilt that no one else will be able to decipher but the two of you.
You're a brilliant little stinker
