Malte hints in 2018 that it will eventually be possible to call MMA functions from SystemModeler, does anyone know how this is progressing. To date the focus has been on adding SystemModeler's capabilities into Wolfram Language.

It is not in general possible (at this point) to call WL functions from SystemModeler components.

I agree that it would be a great feature to have.

What you can currently do is use functions that are simple enough to be converted to Modelica (the language used by SystemModeler models) in CreateSystemModel:

    model = CreateSystemModel[{Unevaluated[Function[#1 + Sin[#2]]][1, 
     u[t]] == u'[t]}, t]

The Scope section of the CreateSystemModel reference page has an overview of what is currently supported.