That sounds doable, but it would not be a huge benefit as the C4D node system would only act as very basic framework to connect A to B - not having significant functionality by itself. Given that the C4D node system is currently separate and different from the XPresso node system, it would not differ much from using a proprietary plugin/external render node system in the first place.
I guess real benefits can only be achieved (long-term) if all nodal systems including XPresso are unified, and all nodes become interoperable on a functional level - a C4D noise node could feed a Redshift texture node, a Cycles color node could be patched into an XPresso vector math node, etc. That way, external renderers really would have to implement only nodes specific to their own engine, and reuse all C4D material and Xpresso nodes that already exist. But the abstraction framework needed to implement this would cause a huge overhead for any practical calculation, so I’m not sure it would be practical. (Not to mention that a render engine may decide to compile its materials internally into some quick-access form, which would be impossible if it had to deal with nodes not its own and go through the framework for any calculatory access.)
What’s more, even if the framework would be completely integrated, we’d still have engine-specific nodes, which means that the great allure of a unified material system fails to materialize. My dream would be that materials become interchangeable (or at least automatically transferable) between render engines, so we would not have to build a material all over if we decide to employ a different engine, and we could take materials from any library to cover our current needs. (I would never expect the final look of the engines to be the same, so switching the render engine within one project / one animation / one distributed render will never be feasible - but a reuse of materials between engines at least doesn’t seem impossible given the fact that most engines follow the same mathematical/physical principles nowadays.)