Hi Vlado,
I asked people over on the Foundry forums and here is their feedback:
“mari api’s are included with the install. The MODO integration source is included as an example of using the render integration.” -- Deke Kincaid
and
"This is an example of what could be done. There is some C Api and documentation for that, and the example of how it works.
MARI 3: MODO Bake and Render Integration" -- netocg
I hope this makes sense and helps. I don’t understand any of it but I hope that means that you have access to an SDK