I was wondering how well these nootropics are absorbed rectally. I'm asking because I remember reading that the anus has the second highest bioavailability, only beaten by IV. These are the substances in question: Centrophenoxine,
Piracetam & Aniracetam.

That is correct, Rectal Bioavailability of "smart drugs" is beaten slightly by IV. However, tolerance will build fast and risk to overdose is high, as the substance is absorbed almost immediately by the large mucous membrane lining of the rectum, and delivered entirely into the bloodstream, bypassing the digestive system. Plugging is considered a safe Route of Administration when dosing is adjusted lower due to extreme efficiency.