Is that not called a register plate.
In my basic reading, researching for a brick chimney for a timber hut, the register plate is often bespoke and made for each application.
As far as I understood, this makes up the horizontal plate which provides a fireproof barrier above the stove AND takes the weight of the structure above.
They are normally made of thick steel with a hole cut to accept the stove flue.
I guess being (expletive deleted) off thick steel, it's no problem to mount any type of chimney liner on top.
No expert, and, not sure I understand but my tuppence worth if it helps😀