Do not dangerously overcrowd the safe-havens.
I would prefer the Fed and the Bank of England charging their respective Treasuries with a commission on all public debt issued. This way the Treasuries would know better that the benefits derived from safe-havens considerations is really not for them to keep; and also that it costs a bundle to keep ever more crowded safe-harbors safe.
That the markets trust Treasuries has more to do with the lack of alternative ports during a very difficult storm than with any intrinsic trust in the harbor chiefs. The governments need to humbly accept that before they and we are left with nothing.