Drawer Slider Setback; 21" slider on 24" drawer

Hello, I'm building a built-in dresser for our newborn's room. The drawer boxes I'm building will be ~24 deep. I purchased some Blum blumotion drawer slides (562F) that are 21 inches long. They're on their way so it's a done deal; this is what I have to work with.

The price between the 24" and 21" is ~ $30 per pair (the 24" are the next quality up; heavy duty) and I figured getting into the back 3" of the drawer isn't worth $30 extra, plus it's a dresser so the last 3" isn't crucial anyway.

Now I'm thinking, what if I were to install a cross piece across the underside of the drawer 3" back from the front to attach the drawer locking devices to. The tracks in the dresser box would be set back 3" as well.

The whole slide system would be recessed 3" allowing access to the entire drawer.

Has anyone done something like this? Can you think of any potential problems installing them this way or should I just install them so they are flush with the front of the drawer box and cabinet as standard?

Thanks!