How are lavatory smoke detectors connected to the flight deck?
Data Bus

A quick Google search suggests the general approach is to minimise wiring by using a data-bus approach to carry information from multiple sources along a minimum number of wires

See ARINC 629 Data Bus Standard on Aircrafts


  In earlier analogue avionic systems the number of cables used to transfer information between the various
  system components was considerable. With these systems, at least one pair of wires has been required for each signal
  and so a typical installation requires several pairs of wires. With the equivalent digital systems, all the analogue signals
  are converted into their equivalent and are assigned unique address labels to ensure there are no conflicts. These
  signals are then transmitted down a single pair of wires, which makes up a data bus. Aircraft data bus systems allow a
  wide variety of avionics equipment to communicate with one another and exchange data. The type of language used on
  an aircraft data bus is known as the protocol. There are currently different data bus standards (protocols) that currently
  account for most of the avionics data interchange on today’s aircraft, and these are: ARINC 429, ARINC 629, MILSTD
  1553, MIL-STD 1773, CSDB and ASCB. 


I imagine that smoke detectors are just one example of the types of sensor that are integrated into such data-bus systems.

I expect there are several data-bus systems for redundancy and perhaps to isolate different categories of data.

Further reading


ARINC 429 as used on  A310/A320, A330/A340; 727, 737, 747, 757 767; MD-11 etc. 
ARINC 629 as used on 777
AFDX as used on A380
Smoke Detectors interfaced by a safety critical aircraft
based CAN-Bus Network