I don't believe this is possible actually. The JSON-RPC connection between MATLAB and PLECS Standalone was a more recent feature we added compared to Simulation Scripts in Standalone, which effectively allow you to do similar things as you would with MATLAB scripts with Blockset. So the motivation was more to provide a similar capability between the two versions of PLECS, but we didn't anticipate a user need for linking the two scripting capabilities. So in short, at the moment you would need to port your logic (with some adjustments for function handles and syntax) from the PLECS Simulation Script to a MATLAB script. We can update this information if this changes in the future.