jgb is correct though - in order to avoid multiple zones sending conflicting signals, each remote in each zone would have to send the appropriate source activating code with each command. However, if all of the sources are in the same place (and use unique IR codesets), then the RT8 is really overkill (I know b/c I've got an RT8 along with the companion RC68+ remote gathering dust!).
If you set up all the sources in one central location, as long as each source can be independently controlled (e.g. multiple DirecTV boxes setup to use distinct IR code sets) then all you need to do is combine all of the zone inputs into one (Xantech make a glorified terminal strip called, unoriginally, the Strip-IR which does this quite well) that then goes into a distribution block (I use Xantechs 10-source amplified block for this), then you're golden. Hook up the sources to each of the emmitter ports on the block and you're good to go. I currently have three rooms hooked up to a 4x4 matrix switch in the basement with IR control wired down to the basement through the Strip-IR=>DistributionBlock=>Sources and everything works great.
If you have multiple boxes with the same IR codesets then you will need to use something like the RT8 to route the IR codes appropriately. An alternative would be to use an RF remote along with an RF-to-IR addressable system. URC makes such a system and, from what I understand, it does the job pretty well.
Hope this helps.