A bit late, but...
The Beethoven organist, IIRC was Wilhelm KRumbach, and he played on an old Rhineland Koenig instrument. There was indeed a sort of "Fugenkreis", and also, I think, Van B's notorious organ trios; the first two are arrangements of movements from CPE Bach's Wurttemberg Sonatas, and the third is an arrangement of a Beethoven Prelude for 2 vln and viola. The French composer Fetis picked these up at Beethoven's estate sale, and that is partly how they travelled to France, and Tournemire ended up editing the three into a volume. Because Beethoven had copied out the first two pieces without identifying them, Tournemire assumed that he had actually written them himself. There is also a very nice fugue, which constitutes the 4th trio.
Maria-Magdalena Kaczor has done a rather good recording on the Aeolus label.
Hope this helps!