copper pipe to introduce secondary air?

Copper sounds like a possible alternative to stainless steel.  It's readily available from plumbing stores, it comes in standard diameters, and it's malleable.
Its melting point and spalling temperature are close together at about 1030 deg C.  If you need more airflow than provided by the diameter from one pipe, just add another in parallel.

Problems?

It is quite a bit more expensive than steel… and i would worry about melting it … (I have melted cast iron already.)

cast iron melts at about 1100 deg C so sounds like you're running a furnace!

I do love a warm oven …

Why not plain old steel or iron? I doubt the extra heat conductivity makes a big difference. (As it needs to transfer the heat to the air.) Okay, bending it requires some force, but it is readily avaible as well.

For me the price of copper is 1/3 the price of stainless steel.

I put my copper pipe on the floor of my batch box and it was fine.
Then I elevated the end that was in the mouth of the Venturi - rested it on a bit of fire brick.
At the end of the build the pipe had sagged, but still otherwise retained its shape.
So, that rules out copper.