You can do this using the External Control Protocol (ECP). See the ExternalControlGuide.pdf in the SDK. Note, however, that launching another channel will close the current, so when the user exits the linked content, they will be returned to the home screen and not the channel that launched it.
My Channels: http://roku.permanence.com - Twitter: @TheEndlessDev
Instant Watch Browser (NetflixIWB), Aquarium Screensaver (AQUARIUM), Clever Clocks Screensaver (CLEVERCLOCKS), iTunes Podcasts (ITPC), My Channels (MYCHANNELS)