Shulgin mentioned in PiHKAL that MD correspondents of Bz-piperazine would be intresting. Here is a patent that describes the process of high-yield synthesis of piperonyl-piperazine by reductive amination of piperonal with piperazine. However it's written in French, maybe someone can translate.
Abstract:
Single-stage preparation of piperonyl-piperazine (I) involves reacting piperonal (II) with a stoichiometric excess of piperazine (II) in a volume excess of a polar solvent until (II) is partially converted into the corresponding hemi-animal, then hydrogenating the mixture in presence of a catalyst.
Patent FR2799464 (http://l2.espacenet.com/dips/viewer?PN=FR2799464&CY=gb&LG=en&DB=EPD)