Maybe I could read the manual, but does an AI Shearwater have a facility to complain obviously in the event of low gas pressure?
It flashes you "reserve pressure" and "critical pressure" warnings. You can set the reserve pressure limit. Critical pressure is either 21 bar or half of set reserve pressure.
A mode where it complained if it didn’t change might be nice, especially if it knew the solenoid was being activated, of course that would need an AI Petrel and soon they’d have built a Sentinel. I suppose a nerd could do it.
Something like that could be built into XCCR with its hard wired divecan pressure sensors. With JJ people use wireless transmitter with Shearwater secondary computers (Nerd or Perdix AI). JJs primary Petrel 2 doesn't read wireless transmitters and a secondary Perdix AI is not connected to solenoid.
Do the transmitters really burn power when the pressure is static?
Yes, the Pelagic transmitter (Shearwater, Aqualung, Oceanic and so on) does. It transmits with a constant interval (about 4 seconds) when pressurized. If it would transmit only when pressure is changing, you'd need another way to validate that the transmitter is alive and connected. It displays a value only if it is recent, otherwise it is interpreted as connection lost.