Hello Retrofit,
I hadn't heard of the Allgemeine Rückkehr Technologie effort before - it's possible that it is a downscaled follow-on to the Arbeitsgemeinschaft Rückkehrtechnologie, which started in 1971 and included universities and the Deutsche Forschungs- und Versuchsanstalt für Luft- und Raumfahrt (DFVLR) aerospace research organization in addition to industry, but perhaps it's just a misinterpretation of the original program. A thread on the Bumerang effort is at https://www.secretprojects.co.uk/forum/index.php/topic,2890.0/all.html.
Martin