I cannot seem to find manufacturer specs. However, this is not a manufacturer-specific interface. It seems to be used universally and globally for all wired interconnected smoke detectors, especially in the states ("orange wire"), and it seems that fire alarms from different manufacturers aso interconnect. The interface is analogue (probably simply voltage or no voltage) so I assume it is very basic.
See URL below for a simple example of such a detector.
http://www.brkelectronics.com/faqs/diy/how_do_i_install_hardwired_brk_alarms
There is no explicit information in the manual.
There are "output" modules from a lot of manufacturers that connect to a smoke detector and switch a relay when the interconnect wire signals an alarm:
This is used a.o. for adding a sirene. You could also attach it to an arduino.
However, I am looking to go the other way, i.e., signal the interconnect wire from the arduino.
I'll try to dig a bit deeper and maybe contact a manufacturer.