Well it wasn't hard at all for me to isolate the source of my problems. Put the MRF into it's RF "sniffing" mode, where the activity LED lights up on all RF signals in the range it works in. You just put the ID switch to 0. Then place it where you get your worst interference (i.e. the LED lights up brightest). Then disconnect the IR blaster cables one at a time. That caused the LED brightness to reduce significantly - which indicates the blaster cables are the root cause.

There may be cases where the incoming DC from the MRF power supply also has RF noise. Putting a very small value cap across the incoming DC should attenuate any high frequency noise on that line. A ferrite core might also be useful here.

But as I indicated, my problem wasn't from the DC power cable. I confirmed this to a large degree by using a 9v battery in place of the power supply to eliminate any possibility of noise coming in from the AC power & through the transformer. That provided NO improvement - no change to my RF interference.

Adding a small value cap to the IR outputs was a possibility, but choosing the right value would be a trial & error process. You would have to find the right value that wouldn't affect the signal to the IR emitter. The ferrites have 0 effect.

One last note... After digging around in my parts drawer, I found the IR cables supplied with my Mitsubishi DLP. Mits has a feature where they can control other equipment via software in the TV. They drive IR blasters directly from the TV. Guess what those cables had on them? Yep, ferrite cores right at the connector to the TV.