Lore wise it wouldn't work either, as Disciples of Magic (with the possible exception of ACN and it's Jobs), use their weapon to focus the aether that powers their spells as a conduit. More so, the weapon actually is part of the casting; thaumaturgury spells are powered through the gemstone that all THM and BLM weapons are encrusted with, and Conjury requires a staff or wand "made of unworked wood" to help channel the natural aether from the environment as well as then focus that conjuration in the form of a spell. Arcanima uses arcane geometry written within the caster's grimoire to evoke spells like summoning Carbuncle (this is why they actually hold their grimoire in front of them and literally fire the spell out of the book).
So lore wise, this prohibits "casting from the hand", hence why this just is not possible I'm afraid (to say nothing of having to change the casting animations as others have said).