The weekend of the Anzac test in April is supposedly an 'international weekend' down under, although this year is the first time anyone other than Aus and NZ have actually played. Spreading this worldwide, you'd then have the problem that this might not fit the UK calendar (Challenge Cup dates in particular appear to be at the mercy of the BBC and there's usually a round of Cup matches near that time). The Aussies wouldn't want it any later in the season as it would interfere with the Origin series, and woe betide that happens.
If full internationals were played, the SL and NRL clubs would have to release the players, but then you have the problem of clubs, allegedly, finding ways round it. The Kiwis always seem to get players pulling out of the Anzac test because they need some sort of minor operation doing or, are suffering a knock of some kind. Nothing too serious of course, as they all manage to recover in time for the next round of league matches. If players were required to fly to the other side of the world for mid season internationals, I suspect the busiest people would be the doctors writing out all the sick notes.
Personally I'd love to see something like it, I'd even prefer us to be hosting a summer World Cup instead of hoping the weather will be kind to us in November. Doubt it'll happen though.