If it's been fitted to other models and been safety tested by crash dummies then there may be a solution, but look carefully at how it was done and if there has been any additional re-inforcement added. If V.W haven't provided it as standard, then you won't know what the restraint will do in a collision.
I edited for this addition: The EOS center seat back is the removable section for carrying skiis. It's a huge hole and I doubt if the structure around it would satisy the crash dummy in a child seat. When I said 'fitted to other models', because the EOS is unique with a removable rear panel, you can only look at EOS variants.