@OP I just recently found out about this -- it appears to do exactly what you (and me) want -- convolution synthesis:
Galaxy X
The KVR thread about it:
KVR: Galaxy X - new convolution synth
It looks like it will be shipping in the next month or two. This might be on an Omnisphere level so I"ll definitely be following its development.
Now that I think about it, it seems very familiar to the Forbidden Planet project that EWQL were working on but never released.