I love projects like this too. I agree that a valve wouldn't work well because you need some level of continuous flow of your wort for your thermocouple to give you continuous feedback unless there is some kind of flow regulator that can be installed. I know nothing of how a variable speed pump works, so I'll leave that part to you. I have a PID in my electric brewing system. The thing you're going to need is fast feed back for fast changes in flow rates. I'm not sure how fast a PID can respond to those fast changes. You'll have to research that. At any rate, at some point, you will find that there is a certain flow rate that gets your wort to the temperature you want, and then you could just adjust it to that flow rate. If you're looking to cool different worts to different temperatures, then you're idea sounds great. If you use a good size plate chiller, you might find that you can push the wort through it at a fairly fast rate and still get to your target temperature. I am currently working on putting together a mechanical system involving a plate chiller. I am going to use a DC pump to push hot wort through a plate chiller and then back to the BK via a whirlpool arm I'm going to build until the wort has reached the desired temperature at which point I will divert the wort to the fermenter. Not nearly as cool as what you're planning.