Usually, the ability to adopt the IOU arrangement is when the trust is established, and not at some later date.
At the present time, the trustees have security for the NRB sum comprising the trust fund. Should they exchange this for an IOU and, at the end of the day, the full amount due is not received they may be personally liable for the shortfall.
Should the trustees subsequently make an appointment in favour of the widow which would have the effect of avoiding liability for any shortfall, it may be open to a disappointed beneficiary to assert such appointment was in breach of trust.
I would be reluctant to exchange the security of a charge with the uncertainty of an IOU, without the protection, say, of a supporting opinion from Chancery Counsel.
Paul Saunders