Wow, good questions Bustr.
Theres been an ongoing development since long time back with this kind of rounds. Since beginning of 70īs. I dont know if 224 toz was the first, but I doubt it. Just look at any wildcat, as this is what it really is. Necked down pistol ammo is no news, Mauser 7,63 (tokarev 7,62) is one and I think that Bergman had a very powerful "export" variant, that was necked as well in early 1900, IIRC. The german 7,92x33 is also something semiliar.
I think they have been down in 4mm as well, but it might seem difficult to get a small bullet like that stabile, it must be some kind of "flacette".
Theres also trails going on with a 6.5 undercalibrated 9mm submachinegun at Bofors for police useage. You can switch ammo while you shoot from 9mm to undercalibrated.