I was waiting to be sure, but it's good: I managed to get my module working!
In 1.7.7.0, my template was not good.
And for the previous versions, you have to add this for the main route, as specified in the doc :
_legacy_controller: AdminOrderPreparation
_legacy_link: AdminOrderPreparation
I got fooled because the demo module didn't have them, except that it's made for 1.7.7.0 minimum so... my bad.
I thought I had already tested with these lines though but there was probably another error at the same time.
I didn't test for 1.7.5.0 because I'm using the new Grid system which requires 1.7.6, so I'm going to upgrade my store to 1.7.6.8.
But I think it should work too.
Good programming to all of you!