This should do the trick:
    _applicationObject.Events.SolutionEvents.Opened += new _dispSolutionEvents_OpenedEventHandler(openedSolution);
    _applicationObject.Events.SolutionEvents.AfterClosing += new _dispSolutionEvents_AfterClosingEventHandler(closedSolution);
"Internal" refference in the MSDN: http://msdn.microsoft.com/de-de/library/EnvDTE.aspx
You can determine the type of the project with this code (from http://www.mztools.com/articles/2007/mz2007016.aspx):
public string GetProjectTypeGuids(EnvDTE.Project proj)
    {
        string projectTypeGuids = "";
        object service = null;
        Microsoft.VisualStudio.Shell.Interop.IVsSolution solution = null;
        Microsoft.VisualStudio.Shell.Interop.IVsHierarchy hierarchy = null;
        Microsoft.VisualStudio.Shell.Interop.IVsAggregatableProject aggregatableProject = null;
        int result = 0;
        service = GetService(proj.DTE, typeof(Microsoft.VisualStudio.Shell.Interop.IVsSolution));
        solution = (Microsoft.VisualStudio.Shell.Interop.IVsSolution)service;
        result = solution.GetProjectOfUniqueName(proj.UniqueName, hierarchy);
        if (result == 0)
        {
            aggregatableProject = (Microsoft.VisualStudio.Shell.Interop.IVsAggregatableProject)hierarchy;
            result = aggregatableProject.GetAggregateProjectTypeGuids(projectTypeGuids);
        }
        return projectTypeGuids;
    }
    public object GetService(object serviceProvider, System.Type type)
    {
        return GetService(serviceProvider, type.GUID);
    }
    public object GetService(object serviceProviderObject, System.Guid guid)
    {
        object service = null;
        Microsoft.VisualStudio.OLE.Interop.IServiceProvider serviceProvider = null;
        IntPtr serviceIntPtr;
        int hr = 0;
        Guid SIDGuid;
        Guid IIDGuid;
        SIDGuid = guid;
        IIDGuid = SIDGuid;
        serviceProvider = (Microsoft.VisualStudio.OLE.Interop.IServiceProvider)serviceProviderObject;
        hr = serviceProvider.QueryService(SIDGuid, IIDGuid, serviceIntPtr);
        if (hr != 0)
        {
            System.Runtime.InteropServices.Marshal.ThrowExceptionForHR(hr);
        }
        else if (!serviceIntPtr.Equals(IntPtr.Zero))
        {
            service = System.Runtime.InteropServices.Marshal.GetObjectForIUnknown(serviceIntPtr);
            System.Runtime.InteropServices.Marshal.Release(serviceIntPtr);
        }
        return service;
    }
You find a list of known GUIDs here
To disable your option you would remove or add the menuentry regarding on the Type (check for GUID) in the openedSolution method