Sure. I suggest to join our #grilo channel in irc.gnome.org.
One of the best ways to understand how Grilo works is implementing a
plugin. Thus you get lot of feedback for what a plugin developer would
need.
There are different services you could write a plugin to deal with.
As suggestion, one of them could be Magnatune. Rationale is that you can
download a full sqlite3 database with all the content. So writing the
plugin would be as easier as downloading that sqlite3 database, and then
implementing the grilo functions on top of sqlite3. You could get
inspiration from podcasts or bookmarks plugins to write it.