I am not sure of the practicalities, but what about a .224 Super or Hi-Power ie the .22 Hi-Power customed designed for a .224 calibre rather than the more limited .228 calibre

Easy enough to do, just a cost and time issue for reamer and dies.

My calculations show that to have a 70 grain 224 at 3000 f/s and very low pressure a case equal to a 30/30 Improved will be required. So a 225 Winchester. I don't think the 22 Hi Power will be big enough. As I remember the 22 Hi Power is similar to the 219 Zipper capacity.

From my perspective if you go for a 224/228 Hi Power then the "classic part" has been lost. Thus if I was doing this I think I would use on of the European rimmed cases and have it in 6mm and use bench rest 68 grainers. These bench rest bullets are quite deadly on the game you mention. Bench rest bullet makers don't thin the nose of the jacket and the jacket thickens at the point during swaging. These bullets tend to go a fair way in and the blow. Talk to anyone who used the 6mm PPC on game.

Since it is to be a double and regualtion is an issue and you want shoot bullets of about 70 grains then to me that spells 6mm all the way. Between the big bullet makers and the custom bench rest makers you have an almost unlimited supply of bullets between about 66 to a 70 grains of so. I think Karl Keuhn in Sydney would do small runs of bullets in different weights. That could help with regultion of a 6mm double.

A 6mm of the same case capacity and using the same amount of powder and at the same pressure will be about 130 f/s faster than a 224. So for 3000 f/s with 70 grains and with low pressure that would make the 22 Hi Power case more suitable.