I always liked how Familiar worked in Baldur's Gate 2, myself.. you summoned it the once, then you let it snuggle up in your backpack in the meantime. Sure, it took up inventory space by doing so, but it felt more 'natural'. I imagine doing that would, in a way, circumvent the unsummoning factor, at least, since by telling your familiar 'get in the bag', it registers which items it has on it, sorta like the warehouse might do, and then saves that info to the 'Familiar' item in your inventory. Basically it'd be a separate PC Info item, dedicated to familiars.
With some of the larger-sized familiars like Hellhounds, you could perhaps say that it shrinks, or transforms into a figurine, while it is in your inventory.