Have not tested it myself, but I believe you can do this with using the ArcGISOnlineConnector. This is using the arcgis python api but no coding needed.

You can also build it yourself if you need something more specific using HTTPCallers / PythonCallers. We use this where we republish Map Image Layers from updated cad files in BIM360 but this is a more advanced process.

​@mattwilkie did you manage to get this working in FME ?  We are wanting to do the same thing.

Interested in testing this ourselves and hearing from others’ experience. This is publishing a referenced editable layer yes?
Understanding reference registered data and copy all data—ArcGIS Pro | Documentation

​@georgiec no we haven’t got this working, but I’ve not spent much time on this part of the workflow since raising the topic. The need hasn’t gone away though. I expect there to be more pressure once our open data portal comes online and we’ll be asked to push more from the backlog through more quickly.

​@crutledge yes, these are referenced layers. Editable? That would be best but we’d be okay with read-only partial solution.

Using ArcGIS Pro I have tried to publish our Enterprise SDE layers out to AGOL as a reference but with no success.  

I’m not sure it’s possible to publish by reference out to AGOL.  Our security would not allow that anyway.  I am only attempting to publish to our Enterprise (internal) by reference.  I’m going to ask our Support people at 1Spatial (Australia).  Will post what I find out.

It appears there’s no publishing transformer / workflow.
 This is the reply from our Support team at 1Spatial:

Unfortunately, this is more of a question to ESRI not FME, but you can start with this reference (of which I'm sure you're aware of): https://enterprise.arcgis.com/en/portal/latest/use/add-items.htm

Just to add, FME can interact with your Portal via ArcPY or the REST APIs once you have services published, so you can use an HTTPCaller transformer to interact with feature services or for administration purposes using the rest apis or Pythoncaller transformer for ArcPY.

We currently use the automatic ‘sync’ on an SDE datastore inside Portal which generates all our services.   Just finding that a little cumbersome and inflexible so was seeking an alternative.