A guy from southern Germany (Johannes) keeps on experimenting with different
kinds of suspension and geometries.
Have a look at this recent video from his thread in a German recumbent board:
There is a front suspension he developed some time ago, but the new part is the
rear suspension using a semi-flexible rope.
With a ratchet system he can wind and unwind it and change seat and steering
angle at the same time - and it is still rideable (OK - the BB comes quite
close to the ground when riding upright).
He started this project from Staiger Airbike and Flevobike geometries, but to
me it looks more like a Python geometry right now.
If you flip back in the thread you see he is really creative in doing things. I
have already ridden some of his full suspended bikes and the comfort is
brilliant - the perfect recumbent for off-road riding!