It appears that the on line Dom wants you to really use diapers for their intended purpose - No if &s or butts !
Humiliation and fantasies aside you really do not want to be exposed at length to fecal matter and urine - onset of diaper rash - which can be a bit painful and uncomfortable. So I would prep the area with lots of Vaseline and when the scenario is over plan to immediately take a shower or bath and clean up asap. You have some ideas on making it difficult to remove the diapers until the required event takes place - Poo & Pee. Whether you want to go for public exposed locking diaper covers or a cover up under ordinary street clothes is something for you to dwell on or in other words just how embarrassing do you want this to be. As for locking or prohibiting access to the diapers you have that to figure out.
Your choice of keyed locks is a simple application making it easy to escape from the diaper prison with the caveat of first thawing the ice that the key(s) are frozen in. You could do what I do frequently and that is use a small luggage or bicycle cylinder combination lock with 3 or 4 rotating wheels. Most of these are designed to set your own combination and the number of combinations is fairly limited so escape will be relatively easy. Putting the combination lock in the front where you can see the numbers and easily dial the combination is not much of a challenge. Putting the lock in the back where the numbers can not be seen now that presents a problem where to solve the combination only the element of feel is available.
As for using keyed locks and freezing the keys in ice cubes (very easy to work with) or ice blocks (now we are getting into something) the factor is the larger the block the longer the time it takes for the ice to form and set-up. Ice cubes in the fridge maybe 4-6 hours all depending on your fridge. Larger blocks of ice will take hours; many hours to freeze solid and you can not just set the key in the ice and expect it to freeze in the middle - this is done with a string to suspend the key in the center of the ice block.
To thaw the ice the most rapid way is heat (hot water) just be careful that when thawing the ice that you do not lose the key down the drain. I am sure that you have heard the ole expression of "throwing out the baby with the bath water".
And of course a large block of ice is going to generate a lot of water to deal with.
You could perhaps think of using locking or restraining mittens to prevent you from using the keys or dialing a combination for release. There was an interesting story awhile back that had numerous keys placed all about the body of the wearer requiring multiple steps to gain the final release from imprisonment. LINK - to the Chinese Puzzle:
http://www.boundstories.net/storiesad/chinese_puzzle.html With this story in mind you could place keys all over the house and then take some time to going to each place where the key to the next key is and so on and so forth.
Word of caution and Warning "Do Not Mail the keys to yourself if they are the ONLY Keys you have" Plan to have back-ups available in case of an emergency that would be more embarrassing than ever ! THINK !!!!! Think medical fire and/or police action - "What you gonna do?" You should in reality have a master or final key available to use in just an emergency.
One place for the master final key would be to tape or secure in the bottom of the diaper you are wearing. You would then have the key with you should you really really really need it. Might take a bit of contortion to get to it - but better that than having no way out immediately.
And then there is always the last resort of just using a knife or scissors to cut off the clothing - but no fun in doing that ! Mamasboy has it pegged keep it clean and do not expose others to the possibility of smell or even a sexual deviate situation in public. And a final thing to think about if you have someone that shares your fantasy.
Then you have a co-conspirator or confederate to enable other possible scenarios such as the both of you locked into individual stages of diaper bondage but needing each other for release before time runs out and the diapers are used for their intended purpose !
It is just so exciting to get worked up about this !
Enjoy and play safe !