The Herald understands her view has the backing of two senior legal academics and at least one leading legal practitioner.
Ms MacLennan said the appointment was a constitutional issue as it could call into question the independence of the judiciary.
But although Mr Finlayson acknowledged the Family Court legislation was "silent" on the issue of a temporary principal judge, he denied the appointment of Judge Ryan was ultra vires.
The intention of the legislation was that there was a Principal Family Court Judge and rather than leave the office vacant he had chosen to appoint Judge Ryan.
"The Attorney-General is aware some practitioners have been trying to stir up controversy around the appointment", he said through a spokesman.
"Judge Ryan was installed as temporary Principal Judge in order to give time to consider an important appointment like Principal Family Court Judge, but also to ensure someone is acting in the position until a permanent appointment is made."
He expected to announce a permanent appointment early next year.
Labour's justice spokesman, Charles Chauvel, said he not only agreed with Ms MacLennan that Mr Finlayson had acted unlawfully, but he believed the appointment was "a cynical political attempt to ensure that the Family Court judges, most of whom are very worried about the effect of the Government's massive looming cuts to Family Court access, would not speak out about those changes through their head of bench".
"After all, a person appointed only temporarily to that position is not necessarily going to feel empowered to speak out against Government policy if the prospect of permanent appointment can be dangled in front of them."
Mr Chauvel said that as Judge Ryan was a validly appointed sitting judge before the appointment, he had no concerns about the lawfulness of any of his judicial activities or decisions since then.
Human rights lawyer Tony Ellis said he was unaware of any legislation providing for temporary chief judges and the appointment suggested poor succession planning.
