I think, for it to work the way that it is intended to work, it must be in the telescope tube, as that is the only way that it could be at a constant distance from the objective lens at all times. If it was in the focuser, its distance from the objective would be different at all different focus points, resulting in altered correction between focus points..
Flatteners can work two ways.
1 - Optical path is static between flattener and image plane.
2 - Optical path is static between objective and flattener.
In case 1, (normal triplet APO flatteners), there is a specified backfocus distance between the corrector and the image plane (usually 55mm). and as long as you're at the specified distance, and in focus, you should have a corrected image.
In case 2, (petzval setups) the specified distance is between the objective lens, and the flattener. And the result of that, is that it does not matter where the image plane is in relation to the corrector, so long as the corrector and objective are spaced correctly, the image plane will be corrected.
You should be able to check this pretty easily by removing your camera from the back of the scope, shine a torch up the focuser tube, and rack it in and out multiple times, you should see the reflection against the corrector, and be able to notice that it is static, compared to the end of the drawtube that is moving.