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But how would one go about writing a  Direct poser integration plugin for a proprietary non-open source engine like Maxwell from next limit or Vray from Chaos Group??

Works the other way 'round. The host application needs the API.  When it comes time to render in, say, Lightwave, the plugin is invoked and via the Lightwave API, the renderer gets "told" what polygons they have, how they're textured, lit and so on.

So Smith needs an API, for Poser, and Maxwell or Chaos would do the work to hook their render engine into it. There's enough experience to know that "if you write the API, renderers will come" -- but you have to write the API. In just the past year, you have the experience of Luxology's Modo . . . they took the trouble to write an API, and Maxwell immediately did a plugin for it. Writing a plugin is not a major chore for a render app, so long as the API is there.

By my lights, Smith (and the long string of prior owners of Poser),  have gone terribly wrong, expending scarce development resources to duplicate what's been done better long ago by lots of folks. Spend your developer resources to give a modern render engine access to the thousands of dollars of content and hundreds of hours of experience I've invested in Poser.

The part of the process that only Smith can do is to alter their code to support an API -- anyone else doing that would be "hacking" -- decompiling or otherwise modifying someone else's code is a DMCA violation.