I had a similar need but couldn't find a way. It was confirmed by the Mathworks tech support that there is no direct way to find the Stateflow chart objects inside a linked library block in a model. I ended up writing the following code. It was verified in many models, including some models with nested library links.
% Updating the model is needed to make sure libraries, if any, are loaded
% There might be error updating the model, but the error are non-critical
% for this operation
try
fprintf(1,'Need to update the model to load all libraries.\n');
set_param(Model,'SimulationCommand','Update');
catch %#ok<CTCH>
fprintf(1,'There are errors updating the model. Ignore ...');
end
ChartRoots=libinfo(Model);
ChartRoots=[{Model},{ChartRoots.Library}];
sfrt=sfroot;
ChartObj=sfrt.find('-isa','Stateflow.Chart');
Index=ismember(bdroot(get(ChartObj,'Path')),ChartRoots);
ChartObj=ChartObj(Index);
