In lieu of Erics' thermometer, can a needle valve (or modified needle valve) be placed into the bolt hole on the front of an E-61 grouphead? I assume it wouldn't seal perfectly, but you may get a fair amount of use out of flow profiling. Any ideas? I'm imagining a nice valve like this slayer knob protruding a little bit out of the grouphead. Perhaps it could be a simple teflon-headed screw that is machined to fit the hole exactly and can be raised and lowered to cover the inlet to the pf?

Eric's thermometer/thermocouple is the feedback component in a system where barista is the controller, all system working under (severe) disturbances
anyway, if you take out of this closed loop system the thermometer from grouphead, the way of monitoring the process inside grouphead, you are losing the control, or you should have another sensor mounted somwhere in the circuit, close to the puck;
if I would have the means, I would try to replace the jicleur in the mushroom, with such a needle valve...

IMO, considering (1) the point of interest, where the water flows before reaching the shower and the puck, and (2) the case of HX machines, (3) during brewing,
there is a relationship between the flow and the temperature of the water: a steady flow during the short interval of 25-30s of the extraction, in a properly designed system, will determine a steady temperature; that is, the heat intake of the water equals the heat loss;
if the flow increases, the heat intake exceeds the loss and the temp will increase, conversely, when the flow decrease, the lesser heat intake will determine a coresponding temp decrease;
added to the above are the high thermal capacity of the group and the high amplitude temperature variations of the water entering into the group from both pipes of the termosyphon;
as a result, an underextraction with high flow would have a whole bumped or sometimes a pronounced ramp profile (the HX extraction temp profile have the initial bump which corresponds to the higher flow during preinfusion with vibrating pump),
while an overextraction, with lower flow would have kind of a concave profile;

bottom line: for E61 HX machines, at least for those with vibrating pump, flow profiling is a tricky experience;
narrowing the scope: for Bezzera Magica, with a flow restrictor zone on the exit of the higher pipe from exchanger, is not exactly the right platform for trying an effective flow control during extraction;

I will stick with Eric's E61 group sensor, wich helped me quite a lot in finding the way to better operate the machine;

... can a needle valve (or modified needle valve) be placed into the bolt hole on the front of an E-61 grouphead? I assume it wouldn't seal perfectly, but you may get a fair amount of use out of flow profiling. Any ideas?...

Im sure it can be done, and would allow some adjustment of flow, however it would also directly alter the pressure that the puck is subject to which may well have other effects on the brew.
..but maybe that is what you are looking for when you alter the flow ?

The thermometer is a must have on HX machines (at least the one I had), but now there is a double boiler E-61 sitting in its place and I was just wondering if this was possible. I don't think this would have too much of an effect on the temperature of the group, since it would only change the rate of extraction by a few seconds.

Yes, the idea is to change the taste of the brew. Or even just to have a 3 bar pre-infusion for longer, or whatever you want!