NCCBR participants are generally subject to regulations and balance sheet constraints - so don't think that null/negative haircuts means the collateral can be rehypothecated infinitely these bilateral arrangements is where financial institutions manage their liquidity needs
NCCBR users are mostly institutions part of the financial market infrastructure - such as dealers and hedge funds they use NCCBR to manage their balance sheets and regulatory obligations. so a high need for Treasury collateral may drive the the haircuts negative