See the question and my original answer on StackOverflow

I'm not sure CPS has anything builtin for this, but you still can use the mix of "old" native/managed Visual Studio interfaces. This is an example that uses IProjectTreePropertiesProvider:

internal class ProjectTreePropertiesProvider1 : IProjectTreePropertiesProvider
    // we need to import that to do COM calls
    protected IProjectThreadingService ThreadingService { get; set; }

    // we want the "old" IVsHierarchy interface 
    private OrderPrecedenceImportCollection<IVsHierarchy> IVsHierarchies { get; }
    private IVsHierarchy VsHierarchy => IVsHierarchies.First().Value;

    public ProjectTreePropertiesProvider1(UnconfiguredProject unconfiguredProject)
        IVsHierarchies = new OrderPrecedenceImportCollection<IVsHierarchy>(projectCapabilityCheckProvider: unconfiguredProject);

    /// <summary>
    /// Calculates new property values for each node in the project tree.
    /// </summary>
    /// <param name="propertyContext">Context information that can be used for the calculation.</param>
    /// <param name="propertyValues">Values calculated so far for the current node by lower priority tree properties providers.</param>
    public async void CalculatePropertyValues(IProjectTreeCustomizablePropertyContext propertyContext, IProjectTreeCustomizablePropertyValues propertyValues)
        // this is from the standard WindowsScript project type sample
        if (propertyValues.Flags.Contains(ProjectTreeFlags.Common.ProjectRoot))
            // etc..
            propertyValues.Icon = KnownMonikers.JSProjectNode.ToProjectSystemType();
            // etc..

        // now, we're doing some COM calls, ensure it happens on UI thread
        await ThreadingService.SwitchToUIThread();

        // get the id of some item (this "Start.js" item is from the standard sample)
        VsHierarchy.ParseCanonicalName("Start.js", out uint id);

        // get IVsUIShell from service provider
        VsHierarchy.GetSite(out Microsoft.VisualStudio.OLE.Interop.IServiceProvider sp);
        var shell = (IVsUIShell)sp.QueryService<IVsUIShell>();

        // get solution explorer's window
        var SolutionExplorer = new Guid(ToolWindowGuids80.SolutionExplorer);
        shell.FindToolWindow(0, ref SolutionExplorer, out IVsWindowFrame frame);

        // get solution explorer's DocView
        frame.GetProperty((int)__VSFPROPID.VSFPROPID_DocView, out object obj);
        var window = (IVsUIHierarchyWindow2)obj;

        // change attribute of the item
        window.SetItemAttribute((IVsUIHierarchy)VsHierarchy, id, (uint)__VSHIERITEMATTRIBUTE.VSHIERITEMATTRIBUTE_Bold, true);