Hi!
I haven't read or followed the discussion, but...
I'd be surprised and disappointed if it is proven "impossible" given the considerable flexibility of Magma's indexing. We probably just need a stroke of creativity or genius to figure it out.
...what about taking a look at say Xapian (seems to be the leading indexing engine) and take a hint from them? Here is one pointer to their new "Flint" backend:
http://wiki.xapian.org/FlintBackend_2fStructure
regards, Göran
PS. Swish-e which we use in Gjallar points to Xapian as indexing engine for the future Swish-e version 3.