Hey there,
As per the snapshot provided by you, I tried to recreate the model using Foundations Library>Electrcial components, a To Workspace block for transferring node voltage sequence to a local workspace named out and a voltage measurement subsystem, whose implementation is described in the second image.
The output voltage of the required node can be extracted simply by connecting the positive terminal of the subsystem to the node itself and the negative one to an electrical reference (ground).
I wasn't able to understand what you meant by dynamically setting the node voltage externally, so I just came up with a model to measure/plot the output voltage of the node you specified.
Then, after connecting the To Workspace block to the subsystem output, I renamed the variable in the block configuration to Vout , which can be accessed in the same workspace/directory using a script file (.m) with the following snippet, where test1.slx is the model name:
TT = timeseries2timetable(out.Vout);
TT.Properties.DimensionNames{2};
outVoltage = TT.(TT.Properties.DimensionNames{2})
I have attached Model snapshots along with the model files (.slx) for your reference as well.