On a personal note, I would agree with Hephaestus; as a GM, I would absolutely allow you to buy a concealed holster for a knife, or indeed any other pistol sized weapon, if you wanted to. Heck, I'd even just double or triple the price for a holster for a sword. Shadowrun has all kinds of gimmicky tech to make this happen, and previous editions even had chameleon coating you could apply to a rifle sized weapon to make it (almost) invisible.
I would not be surprised if this is an item that makes an appearance in future publications.