The plastic seal on the back of the group has worked nicely. After a period of time it did weaken, so I custom fit a stainless washer (drgary suggestion) to it as a backing and kept it in place for several weeks. It was a good method of controlling the group temp.
NOW I HAVE A NEW HEAT CONTROL METHOD. SATURATED GROUP IN ACTION.
Taking a hint from rpavlis's post I ran across this part for $5+ on theespressoshop:
This part is for the La Pavoni lever v3/Millenium group. Theespressoshop measured the part for me, and I decided to order one and fit it to my v2 pre-Millenium model.
Once the part arrived I attached a bolt through the center, put that in a drill and then used a file against the spinning part to cut down the threads and the part size to fit the back of the group exactly. It was a very tight fit, but a bit of tapping put it right into the group. The existing dipper tube fits the back of that part, screwing in as usual.
Worked, but it failed... The part, after heating up, seemed to loosen and only stayed in place for 1 session.
It worked great though.
Back to modification I used a round file and with the spinning part in a drill I cut 2 grooves for o-rings. It took some persuasion, but after some work I got the part back into the group and have been using it a few sessions without any problems. Seems very solid in the back of the group right now, so I hope it stays that way. A lathe should be able to do all this much easier.
Increasing the boiler pressure to 1-1.1BAR I can get the narrow front of the group to 100+c with a few pumps of the lever and watch it slowly drop to 90c if I want a lower temp. With the boiler pressure at .75 bar my max temp was around 96c. The outside flare temp is about 7-10c less than the narrow of the group, so shots aren't overheating even at 100c it seems.
The main drawback to this is that I'll probably always have group thermometry in place to use it effectively. Then again I'm pulling 8 back to back shots without any problems, so I'll accept that as a requirement for now.
Today's experiment with the higher group temp of 100c was better than yesterday's 95c max shots which were a little sour. The naked PF goes in at room temp, so it's likely helping to keep the shot from overheating too. I'm pretty happy with the new modification and temp stability, and feel it's better than the previous plastic steam seal I used.