I'm not sure if perlite would add anything to aircrete.
Perlite is cheap and full of air, but not as cheap and airy as a soap bubble!
The peg and slot idea is good one.
I to help anchor the pipe would probably wire mesh to it,or run self tapping screws into it,or both.
Metal,even buried in the 'crete, could corrode significantly.
Basalt rear and mesh would fix that, but I prefer locally available matirials,and I can't find it around here.
Thus far I have lined a junked file cabinet with the slabs, using neat(strait) cement to merge them with each other to form my firebox.
I used some inserts in the forms in an attempt to form details like the port.
Next time I will just use my handsaw.