We've had this discussion before.
I can speak for Germany only, but here in Germany, our breeding associations will allow AMHA registered horses only. This is partly due to the fact that (until recently) hardshipping into the AMHR was pretty cheap and pretty easy and the association wanted to prevent people from hardshipping their horses with pure German or European background into the AMHR only to call them "American" Miniature Horse afterwards which they clearly weren't. The AMHA on the other hand acted pretty quickly when a motion was made to disallow hardshipping horses with no American background.
And even though we will eventually have to worry about what happens to the oversized horses, I don't think that there will ever be a real market for R-horses in Germany. The reason is simply that in Germany, 86 cm (34 inch) has always been the mark for "miniature" horse breeds (Mind you, in Germany, there are plenty of different small horse breeds with each breed having its own studbook PLUS partbred studbooks) and 38 inch or under is just not a size that has any meaning in Germany.
I do think that there will be a chance that R-horses or oversized horses may be allowed to be registered into one of the partbred studbooks (even though a recent motion to allow that was dismissed), but they will probably never get their own studbook as AMHA horses have. It is not ultimately necessary for a German breeder to have his horse registered in a German studbook, but it increases the value of the horse.
That is not to say that we don't want or don't need rules for what happens with oversized horses, it just means that it isn't easy for us to work everything out. But we're doing our best.
Cheers
Julia