Let's suppose you have that in .idl:

interface ITheComObjectDispatcher : IDispatch
{
    HRESULT TheMethod(BSTR secret, [out, retval] BSTR *pOut);
};

This will become something like this with .NET's tlbimp (as you have seen):

[ComImport, TypeLibType((short) 0x10c0), Guid("D4089F1D-5D83-4D1C-92CD-5941B35D43AA")]
public interface ITheComObjectDispatcher
{
    [return: MarshalAs(UnmanagedType.BStr)]
    [MethodImpl(MethodImplOptions.InternalCall, MethodCodeType=MethodCodeType.Runtime), DispId(0x60020000)]
    string TheMethod([MarshalAs(UnmanagedType.BStr)] string secret);
}

So you cannot use this directly if you don't want to use unsafe string arguments. The solution is to redefine the COM interface in C#, like this for example (note the InterfaceIsDual option):

[InterfaceType(ComInterfaceType.InterfaceIsDual), Guid("D4089F1D-5D83-4D1C-92CD-5941B35D43AA")]
public interface ITheComObjectDispatcher2
{
    [DispId(0x60020000)]
    IntPtr TheMethod(IntPtr secret);
}

And use it like this:

var comObject = new TheComObject();
var comObjectDispatcher = (ITheComObjectDispatcher2)comObject;
var ptr = Marshal.SecureStringToBSTR(mySecureString);
try
{
    var outPtr = doc.TheMethod(ptr);

    // get the output string
    var output = Marshal.PtrToStringBSTR(outPtr);

    // free the callee-allocated BSTR
    Marshal.FreeBSTR(outPtr);
}
finally
{
    // free the secure string allocated BSTR
    Marshal.ZeroFreeBSTR(ptr);
}