Well... I don't know if this is the "smoking gun" you are looking for, but here
http://www.autoelectric.ru/auto/volvo/740/1989/740-89.htm is a series of electrical schematics for the various ( bosch and regina) systems. Look at item # 187 (O2 sensor) for the various Bosch injection systems, and then look at the Regina. The Bosch sensors have a separate circuit for the signal feed, and one for the heater. The Regina's are connected ( signal and heater) internaly to provide the base signal voltage for the ECM.
When I first went to test the O2 sensor on the 94 940 by allowing the car to come to running temp then unplugging it, my first reaction was WTF! No volts, dead. &*%$! But then I remembered the 88 Jeep with a Titania type I had worked on a while ago, and for giggles put the DMM on continuity. It lives! I was getting correct ( for a Titania) resistance swings. Then I hooked it back up and then back probed the connection. Correct voltage! This one is original with 135000 miles on it,
and after changing the plugs today (that had been in 30k that were nice and clean with just a small amount of light tan residue) is still doing a resonable job. The Regina sensors may be a PIA to locate, but they seem to be long lived.