Thanks for the input.
I also do not think this is a real standard.
However, there seems to be a large family of mainly identical IC's that powers most of these optical detectors. See below for one of the data sheets.
The interconnect seems to be a relatively dumb I/O pin:
This pin can be used to connect up to 40 units together in a wired-OR configuration for common signaling. VSS is used as the return. An on-chip current sink minimizes noise pick up during non-smoke conditions and eliminates the need for an external pull-down resistor to complete the wired-OR. Remote units at lower supply voltages do not draw excessive current from a sending unit at a higher supply voltage.
I/O can also be used to activate escape lights, auxiliary alarms, remote alarms, and/or auto-dialers.
As an input, this pin feeds a positive-edge-triggered flip-flop whose output is sampled nominally every 625 ms during standby (using the recommended component values). A local-smoke condition or the push button-test mode forces this current-limited output to source current. All input signals are ignored when I/O is sourcing current. I/O is disabled by the on-chip power-on reset to eliminate nuisance signaling during battery changes or system power-up. If unused, I/O must be left unconnected.
In your link, the above is reffered to as "the original "dumb" smoke detector interconnect".
Your link seems to decribe a much more advanced system of kiddle where they run a more advanced digital protocol over the interconnect wire to distinguish between alarm conditions.
I'll dig a bit deeper, and maybe try to get some smoke detector schematics. The interconnect system seems to be very simple at first sight. Deriving the 9 VDC voltage from mains is a slight complication however.