It has been known for some time that certain European clubs covertly pressure Tier 2 players to make themselves unavailable for international duty under pain of losing their jobs, and they are told that if they talk about it, that will lose them their job as well. This has been going on since the 1990's; the trouble is that knowing it is one thing, and proving it is quite another, and you have to prove it before you can enforce the regulations.
IMO, there can really only be one definitive, comprehensive answer to this and that is for WR to take control and create playing windows in the regulations for both Elite Domestic and International Rugby - no international rugby to be played in the elite domestic window, and no elite domestic rugby in the international window. Then create a third "elite rest" window (or perhaps two shorter elite rest windows) and no elite rugby at all to be played in those windows.
ALL matches in Elite Domestic competitions such as Super Rugby, Top 14, Aviva Premiership, ProD2, Pro12, ITM Cup and Currie Cup (including pre-season m,atches) must be played inside the appropriate defined window. If there are more matches than will fit (I suspect Top 14 and ProD2 would probably be the only one affected) then they have to take a leaf from NZRU's book and do what we do with the ITM Cup, where each team plays a match mid-week with a short turnaround so that the 10 rounds of the regular season can fit into 9 weeks. It works for the ITM Cup, so there is no reason whatsoever that it cannot work for any other competition. If any domestic competition wants to expand, they will have to do so within the constraints of their window.