Now you can use HAL to play music through SlimServer. I'm still testing it out, but it's working pretty well. It extracts all this info out of the SlimServer database and creates the voice commands that allow you to queue up music by artist, album or playlist. Click the screen cap below for a video of it in action.
Even cooler is it can be extended to support other media servers through the scripting engine. There's an option to just pass to the scripting engine the zone, category (album, artist, or playlist) and selection that were chosen. This way the scripting engine can issue a command to launch another player. Granted you'll need to have SlimServer installed but not use it for other than its database.
This works for us since we use SlimServer for our SliMP3s and Squeezeboxes and MediaNet for our Rio Receivers. So for MediaNet zones, xPLHALi will pass parameters that the scripting engine can use to issue xPL commands queueing up the desired music to those non-Slim zones.