I just tried this with the superglue method, and here is the finished result:
The seedling is grafted onto a single phylloclade with a good root around 2" long.
This was a little rooted plantlet in my main pot of Schlumbergera, and this phylloclade had developed a cylindrical shape.
I cut off all the branches, leaving just the cylindrical one, and then cut off the top of the Carnegia seedling.
Then, I shortened they phylloclade, and pressed the two together.
I superglued the edges.
This will be a temporary stock.
I will graft it onto Hylocereus eventually, and while this plant is growing on the Schlumbergera, my Hylocereus seedlings should be maturing.
Once they Hylocereus is ready, I will de-graft the Carnegia and re-graft it to Hylocereus until it is around 2-3' tall, at which point I will permanently de-graft it.
I'll post my progress here.