One excellent place to ask is over at the John Bridge Tile Forum -- here are a few threads discussing methods you can use to affix the Kerdi to the cement block with some suggestions on other materials & techniques that might work as well, depending on your specific situation:

https://www.johnbridge.com/vbulletin/showthread.php?t=69342

https://www.johnbridge.com/vbulletin/showthread.php?t=108221

https://www.johnbridge.com/vbulletin/showthread.php?t=131406

If you can't find the answer you need, I warmly recommend joining and just asking, showing them your photos - in my experience the tile professionals there are extremely kind and helpful. Best wishes on your project!

PS: hi weedy! I seem to remember you from gardenweb days of yore. :)

Pull the window trim, stool, and apron, and build out the jambs the thickness of the Kerdiboard. Glue it to the masonry.

Full bond.. floated w medium bed mortar the CMU must be clean and have no sealer on it

OK, finally starting to plan the work for this room. The plaster is very firmly adhered to the block, so it's adamantly expressing its desire to stay.

Can I tile directly on the plaster? This would be back wall only; ends would be Kerdiboard. Paint a membrane over top of it?