ALL cars by regulation have a J1962 socket which may be used by ANY person with a scan tool to read emission related error codes (DTC). It is illegal for a manufacturer to bar access to said codes, thus the commonality with all cars. Said codes are designated by standards whereby ALL manufacturers have to comply. For instance a code P1313 will mean the same for a Chrysler or Volvo - namely Misfire rate catalyst damage bank 1 (A).
Non emission related codes are not covered by said legislation, so without the aid of GT1 one could not determine let us say ABS codes / errors.
Quite how motorcycles get round this state of affairs is unknown to me, but I would suggest that the sctrictures placed on motorcycle emissions are not as tight as that for cars. However, I would suggest that the error codes are industry standard, but the access ports are not; hence no generic scan tools.
Regarding CAN, if I may clarify? The acronym stands for Contoller Area Network and does not relate specifically to the 'twisted pair' connecting harness. The CAN protocol allows connection of nodes to report to, or read from, the ECM. Nodes being hardware such as knock sensor, pedal demand potentiometer, IAT, AC compressor state, etc, which relate to each other as required by 'packets' of bytes. The ECM for instance needs to be aware that the AC compressor has engaged, thus requiring an increas in idle speed. The AC sends said signal on the network for all to see, but only the ECM is affected and acts on said state by opening the throttle to aquire the necessary idle speed (same thing happens when increased electrical load is placed on the system). This is why you have idle control valves on the 1200 GS to maintain the correct idle speed, thus a stable charge rate and emission output. Things will be much better when full 'fly by wire' is fitted.
My apologies if this sounds pedantic, but I hope that it may clear up some misconceptions.