Hi @sonic30101. I finally got around to trying this cable splicing idea as you suggested. I took a stereo cable, spliced red on one side to white on the other, kept the yellow together, and left the other red and white capped, and plugged the red side into the transmitter and white side into the C4. Unfortunately, no luck.
Any other thoughts?
I guess I'm going to have to give up on the HDBaseT matrix and just bite the bullet and run another HDMI cable from the AVR to the projector.
Thanks!