Soda blasting is non-abrasive
http://www.sodablasting.com/applications/category/engine-parts/
Plenty of people in the states/ Oz reccomend it as a safe and cheap option.
Yeah, I have water injection on my veg burner, amazing how the crankcase pressure can be reduced with 5lts water and 50 miles.
I made a nifty, if dangerous, brush for cleaning out waterways. Get an old speedo cable, cut about a foot of it off and splay one end like a sweep's brush, put the other end in a drill..
How have you achieved water injection?