I have such nervous and excited butterflies for you!!
First, your girlfriend is sure to love her thoughtful and magical proposal.
When you approach the security tables, try to nonchalantly have your girlfriend go to a different table or have her go in front of you and tell her to wait up for you after the tables. You could also suggest that she go ahead and get in line at the turnstiles. Ya know, just to save time!
At any rate, if you have the book and ring hidden well, it shouldn't be revealed. The security people usually do not make guests take any items out of their bags. Just do not have your future fiance standing behind you where she could peak inside! While my heart is racing for you as I imagine you standing in that security line, I think you'll be okay. If you have any concerns, feel free to visit Guest Relations (alone) to inquire about more ideas.
I recently saw a couple get engaged at Cinderella's Royal Table. The girl looked completely surprised so somehow her Prince Charming kept that ring hidden!!
You can see the calendar
here. Click on your intended date to see the Wishes fireworks schedule. To remain in a park or enter a park that is hosting Extra Magic Hours, you will need to be staying at a WDW resort. In order to take part in EMH, you'll need to show your Resort ID (Key to the World card from the hotel) and then you'll receive a wristband.
Best wishes to both of you for a lifetime of magical moments!!