I do not think you want to regulate that. Let the heat flow.
I was thinking you might be setting up something like a marine water heater that has a preheat loop from the engine. This way you would not need to run the engine to temperature before drawing a shower from it.

If you want to control the temp at the outlet I would use an antiscald valve as used in house plumbing to blend hot and cold to give a set max temp.

Cheers, Charlie
If It ain't broke, Modify it!
87 Montero turbo Converted back in Spring1989
95 Montero SR 3.8 DOHC Only one?
93 Pajero 3 door 6G75 Mivec with paddle shifted 5 speed
Then a Gen2 SR with full coil independent suspension.