I'm not saying it's impossible, but consider -
The Garand bolt serves as the "lid" for the en-bloc, keeping dirt out. It strips a round off the ears of the loader, and a tricky latch permits the clip to eject upwards when it is empty.
The main problem is spring direction. You load from the top, compressing a spring that feeds rounds upward toward the bolt. The magazine follower moves in a slot at the front of the en-bloc.
For a bottom-loading en-bloc, you have to provide a similar mechanism for spring tensioning, but you're upside down. All I can imagine is some kind of removable follower / bottom plate (which would also keep out dirt), that you would insert after the enbloc.
With all this in the way, I can't see how ejecting the enbloc on last round would even be feasible.
Someone else who knows more about Garand history can provide details, but I think the whole reason for the en-bloc was military mindset against removable magazines. Once that bump was passed, there was no reason to return to the cleverness of the en-bloc.
I believe, the more you sketch some kind of bottom inserting en-bloc, the more the result is going to look like a removable box magazine...