Very interesting thread! These are all great ideas! But, I have a technique that I love, and want to offer as a possibility - cyanotype printing of fabric.
Basically it is the same theory as how a photographer develops his pictures - treated fabric that you expose to light. I have 2 albums which show this (
http://www.quiltingboard.com/members...-albums81.html &
http://www.quiltingboard.com/members...-albums82.html) . I have done quilts with photos and writing.
You could then just print squares of special photos, a few scattered love letters, etc, then have one square with the proposal. That way you would still be making it all, but could have some simple patchwork squares to sew - to simplify the process for you, since you have never made a quilt before.
If you want more help on this process, need to know where to buy supplies, or just want to do more research, feel free to PM me (just click my name, it will forward you to a messaging page).
No matter what you decide - good luck with the quilt, be sure to show us (even while it is being made - we all love to watch a quilt in process), and congrats on the engagement (how could she resist such a sweetie?).