> just curious, what is the purpose of the screw on top of the grouphead?
Purely for manufacturing reasons. They drill a water path into the grouphead (instead of making a complicated casting form) and afterwards have to close an unneeded hole. This picture should explain it.
> Could it be used to monitor temperature, or maybe pressure?
The position is not advantageous.
If one wants to get as close as possible to the real temperature in the filter, that point will not do. For a good measurement, drill straight down through the grouphead and through the shower screen screw. A 0.5mm thermocouple inserted that way (installed with a compression fitting) ends in the coffee. Example: https://www.kaffee-netz.de/threads/installation-eines-temperatursensors-in-silvia-bruehgruppe.111093
For pressure measurements, it does not matter (noticeably) where the pressure sensor is installed at the boiler. It's not necessary to get as close to the filter as possible. It's easier to install the pressure sensor - which is quite long if installed with plug - at another point, e.g. with a 1/8" T fitting at the boiler input port.
If you are interested in installing a pressure sensor, take a look in the appendix "Pressure sensor installation" in the installation pdf of the leva! firmware. leva! adds pressure profiling to machines.