Boiler cold water return - thermal shock

It's called Boiler Return Water Temperature Protection.

You can use an LK820 valve: http://www.lkarmatur.se/Documents/LK Armatur/Instruktioner/048313 LK 820 1 st.pdf
Cost around $125

You can use an LK810 pumpgroup: http://www.lkarmatur.se/Documents/LK Armatur/Instruktioner/048302 LK810 Laddpaket se, fi, eng,de,fr, it.pdf
Cost around $625

Or you can use your injection pump concept

The guys in the boiler room can give you much more input.

Our house is a converted gravity system with big pipes and cast iron rads that are over-sized now that the place has been tightened up. So when zones do activate they've normally been idle for a while and there's a big slug of cold water. With the boiler aquastat in the top of the oil boiler water jacket, by the time the boiler aquastat could see the cold water the boiler was shocked and chilled.

So we added a boiler loop with injection, just as you've drawn, and as Marc has identified as a possibility. I added an aquastat on the boiler return to disable the load circulator whenever boiler return is below 160 degF (or 150 or 140 if you prefer). With a 1.25 inch boiler loop a typical 75 watt circulator will flow well over 15 gpm, which tempers the cold return water nicely.

But when I did mine I couldn't find a thermostatic mixing valve for less than $250, and I had a salvage pump and aquastat laying around. For $125 the mixing valve Marc identified looks like a nice way to go.