Greetings! @Danielst,
Are there any sources for the explanation that asanna realm and arupa brahma have hadaya vatthu?
From the book A Comprehensive Manual of Abhidhamma, in the Compendium of Matter, page 257-258, it is stated that the Non-Percipient Beings (Asaññasattā), the eye, ear, heart-base, and sound are not found. Similarly, no consciousness-born material phenomena are found. Therefore, at the moment of their rebirth-linking, only the vital nonad (pure octad [eight inseparable material phenomena] with vitality [life faculty]) arises. During the course of existence after rebirth, material phenomena produced by temperature, with the exception of sound, continue.
The book does not mention anything on the arūpa brahma.
With mettā, Seng Kiat