Without some really intricate hydraulics, you wouldn't get any useful information on the brew circuit by measuring it with a gauge that only reads up to 5 Bar. I'm not sure how you'd plan to do it, but you'd be better off using a gauge that can measure up to at least 10 bar (like a brew pressure gauge) and reading the pressure right there in the head, above the puck but after the gicleur.

Assuming you're talking about your dual-boiler machine... If activity in your brew circuit is directly affecting your boiler pressure, you need to get that fixed.

If you're talking about some SBDU machine with a gauge, then yes, that would be a pump pressure gauge and it should go up to at least 12. If you found a way to have it reading the extraction pressure by having it read pressure above the puck but after the gicleur, then yes a gauge just like the pump pressure gauge would be fine. It would be far easier to use the gauge that comes with the flow control in the proper hole and have the temperature read elsewhere. It may be easier to build a custom, hollow-threaded shaft for the hole in the E61 head through which the leads for your thermometer's NTC sensor pass but also through which pressure is monitored by a gauge on the end of the threaded shaft.

I updated the title of the thread to refer to the machine's brew pressure gauge and not boiler pressure. I hope it clarifies what I'm talking about. In my machine this gauge stops at about 9 bar when backflushing, but can obviously show more if the OPV is set differently.

It's accurate enough, yes. If you can figure out how to have it reading pressure in the brew chamber, it will work.