Hi Dan,
Welcome to the club.
I think a 1/4 thickness is what would be most common for an exotic hard wood solid body electric guitar top. This may also be dictated by the thickness available from the wood supplier, unless dad has an industrial planer that can turn any board thickness to dust. Practically any thickness would be feasable. Have you considered a set neck? I know that bolt-ons may be easier to manufacture, but a set neck when glued to the body becomes one with said body, and imparts an altogether unique and specific tonality. Alembic even has a few set-neck models, and I for one can attest to their sonic abilities! Are you going to put Alembic electronics in the guitar? The Activator circuit is really top quality and furnishes the classic Alembic low-pass filter sound. Available in a few different classic pick up configurations, and also a few different control options. Worth considering for any boutique custom built guitar!
Keep us posted, and especially with pics!
Kris