I wouldn’t recommend this.
While your custom node might be able to multi thread, the subsequent and proceeding nodes will not be able to do so. Unless your one node is obnoxiously long to run, I don’t think you’ll get much value there. Most (if not all) geometry operations in all CAD software (not just an Autodesk thing) today is single threaded for a reason: The intended gains actually set you back more in the end.
Further once you get into Generative Design you will be utilizing every core on the system concurrently to it’s fullest extent. I typically see 112% CPU usage for big runs (the extra 12% is the service CPU which deals with managing the threading). By incorporating your own multi-threading you’d break the multithreaded gains which are built into GD, and risk corrupting you’re study data as the CPU could lose track of which thread of data belongs to which study.
API is a vague term. Since GD runs Dynamo sandbox, anything accessible there will run (ie: the Dynamo API). Anything which requires accessing an integration (ie: the Revit API). Generally speaking I recommend keeping the data crunched by GD in the out of the box sandbox toolset, removing external dependencies as much as possible.