I know this is an old thread, but I recently confirmed with Brussel's about their current soil blend.
I bought three entry-level potted bonsai from them last month and they all use an updated soil blend (looks nothing like the soil in the original post).
Their current blend is 50% "high-fired aggregate" and 50% pine bark. Probably still not great, but an improvement vs what they seemingly used to use and ship their bonsai in. Not sure about he exact identity of the aggregate (I'll update if they reply to my question).