Based on this Intel sourced info linked below, it states that you can run dual channel when you configure the ram on a four-dimm machine similarly to what you are thinking about doing on your six-dimm motherboard. It seems very plausible to me that if you followed the same method, but instead populate 4 out of the 6 slots that you might get tri-channel.
If it doesn't work then you could populate the slots in such a way as to utilize them in dual-channel (24GB) with very little performance loss when compared to tri-channel.
Lastly, there is the "flex-mode" option (read last part of the linked page). You would at least get the additional ram that you're looking for and run all 5 modules (28GB)
I keep meaning to test these various alternatives but just forget to do it when I pull out my X58 rig. Gotta write myself a big note, I guess.