@LilliSmith can weigh in here, but I am fairly certain that you can drive a mass family from Dynamo. In place massing is a no-go as there is no such thing as in place API - families are more robust, practical, and reliable anyway, and can be edited after the fact. You’ll want to look into sending the component elements (the adaptative points which define the curves which define the surfaces which define the mass), and having the API generate the content from there. It’s an involved workflow, but is valuable once you learn it. I cannot share code samples directly as they were built for a client, however I can provide guidance in another thread once you start developing something.
If SAT works, you can also export out directy to SAT from Dynamo, and either manually import the files or write a graph to do the imports for you in bulk.
External links, sure - just not ones which involve the Revit API. More on this in a bit.
To work with Revit content requires a Revit instance that isn’t actively processing another command. Generative Design spins up six instances of Dynamo sandbox concurrently, and uses each of these instances to run a copy of your graph with the inputs set by Generative Design. As such, even if you could hook into Revit you wouldn’t have an idle copy which isn’t processing any data - there first Dynamo sandbox build would be tying up the subsequent ones.
But note that I did say external links are possible. Dynamo core can read in external data from a variety of sources - you acn see which these are by reviewing the content available in the Dynamo Sandbox. Excel and CSV to move numbers are well known examples of this. However you can also build your own framework for serialized data in these formats - JSON is a common means of moving such data around, including Geometry elements.
Setting up a design option, and having Dynamo place the family instance in Revit will work. However this has to happen after a Data.Gate node. You can read more about those in the Generative Design Primer here: Using Data from Revit - Generative Design Primer.
In the latest build (Revit 2023) you can actually export the results directly from inside Generative Design - no need to look into the history which nick mentioned if you get current. 