I think the simplest way to get what you want is to extend VelocityView and override the mergeTemplate method as follows (UNTESTED);
Code:
protected void mergeTemplate(Template template, Context context, HttpServletResponse response)
throws Exception {
// add self reference to the context
context.put("velocityContext", context);
template.merge(context, response.getWriter());
}
Then your render tool can access the context itself with the name "velocityContext".
Let us know how you get on.