I recently found the need to display a plugin dynamically, in a very similar way to the ‘Form Data’ plugin, in a certain PropertyData type existed within a page type. After reflecting the code for the form data plugin I was able to duplicate the functionality:

Firstly the plugin class needs to inherit from ICustomPlugInLoader, which exposes the method List()

public PlugInDescriptor[] List()
{
    if (PageAllowsComments())
    {
        // Show plugin if comments allowed
        PlugInDescriptor descriptor = PlugInDescriptor.Load(GetType());

        return new PlugInDescriptor[] { descriptor };
    }
   
    // Hide plugin if comments not allowed
    return new PlugInDescriptor[0];
}

private bool PageAllowsComments()
{
    PageData page = GetCurrentPage();

    if (page != null)
    {
        foreach (PropertyData propertyData in page.Property)
        {
            if (propertyData is CommentsXForm) return true;
        }
    }

    return false;
}

private PageData GetCurrentPage()
{
    PageData currentPage;
            
    if (PageReference.IsNullOrEmpty(base.CurrentPage.PageLink))
    {
        return null;
    }

    try
    {
        currentPage = base.CurrentPage;
    }
    catch (EPiServerException)
    {
        return null;
    }

    return currentPage;
}