I'm not sure why they have to be high-profile people (therefore demanding a high salary).
Why not make it a job for the outgoing ACT Mayor or some other standard reward? Since they get a free house, how about just a $350k salary. I'm sure they could make do.
Most of them don't even bother to live in the free house they're given, they're so important.
I wouldn't know enough about former Canberra Mayors to comment on whether a municipal official would have the required expertise. For me, Ninian Stephen and William Deane are the gold standards. Former High Court Justices have the required expertise in constitutional law.
They don't have to be high profile: most people couldn't name a current High Court judge, me included.