In the end, I extended the HtmlHelper in the same way as Core does. The gridrendering code now has access to a GridContext as well as the Grid itself.

public class GridContext<T>
{
    public IPublishedContent ContentItem { get; set; }
    public string PropertyAlias { get; set; }
    public string Framework { get; set; }
    public T Args { get; set; }

    public GridContext(IPublishedContent contentItem, string propertyAlias, string framework, T args)
    {
        ContentItem = contentItem;
        PropertyAlias = propertyAlias;
        Framework = framework;
        Args = args;
    }
}

public class GridModel<T>
{
    public GridContext<T> Context { get; set; } 
    public object Grid { get; set; }

    public GridModel(GridContext<T> context, object grid)
    {
        Context = context;
        Grid = grid;
    }
}

public static class GridTemplateExtensions
{        
    public static MvcHtmlString GetGridHtml<T>(this HtmlHelper html, IPublishedContent contentItem, string propertyAlias, string framework, T args)
    {
        Mandate.ParameterNotNullOrEmpty(propertyAlias, "propertyAlias");

        var gridModel = new GridModel<T>(new GridContext<T>(contentItem, propertyAlias, framework, args), null);

        var view = "Grid/" + framework;
        var prop = contentItem.GetProperty(propertyAlias);
        if (prop == null) throw new NullReferenceException("No property type found with alias " + propertyAlias);

        gridModel.Grid = prop.Value;
        var asString = gridModel.Grid as string;
        if (asString != null && string.IsNullOrEmpty(asString)) return new MvcHtmlString(string.Empty);

        return html.Partial(view, gridModel);
    }
}

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