With Alembics, I would not worry a lot about the neck doing tricks whilst unstrung. The 1/4" ebony fingerboard plus the laminations are generally plenty stout and tend to not move a lot, barring the occasional brand-new build that is still a little flexible, relative to your older guitars that have stabilized to a mature state. I've had less complex necks that were flexible flyers (Fender-style maple on maple necks . . . . ), but that's a long way from your Alembics, structurally.
Strangely enough, you can kind of test that flexibility . . . . . chime a harmonic chord, then reach around and press the back of the head to see if you can get a tremolo like wobble. If you have to press like hell to get any wobble at all, it's really stable, and it's usually hard to get an Alembic to do that. Not scientific at all, but indicative.
Joey