The bivert module I bought from handheldlengend.com has some design flaws, the unused inverters aren't grounded and there is no decoupling capacitor. So for future mods I will use one of these boards that feature a dual inverter named SN74LVC2G04. It's also smaller and potentially easier to install in the Game Boy Pocket.

UPDATE: there is a smaller version for Game Boy Pockets to buy already at http://store.kitsch-bent.com/product/gb-pocket-lcd-inverter

UPDATE2: a redesign with an dual XOR chip might be interesting, to revert the bivert. Sounds like a dance move :D