There is no easy solution for this. Nothing available from the theme or [show_posts]. All of these are designed to work exclusively on a single site.
The only simple solution I can think of is some kind of specialized feed from the subdomain, or linking to a page on that site. But actually displaying content from a different site is not at all easy. There is no difference at all between any old external site, and a sub-site on your multi-site. Different URLs, different site!