Some motorcycles have a CAN BUS, some don't
The OBDII connector type is simply a standard instituted by the SAE (Society of Automotive Engineers). In 1996 the US government mandated that all cars sold in the United States conform to the SAE OBDII standards. They didn't specify motorcycles. As result, a myriad of on-board diagnostic methods were implemented. Some failed and some rose to the top.
They may not have an OBDII connector but many motorcycles still have a CAN BUS.
To answer your question.
What is the motorcycle equivalent of OBD-II?
Essentially, many motorcycles conform to the CAN BUS protocol they just don't have an OBDII connector, they will have their own proprietary connector and you can buy converters for the various manufacturers that do support the protocol standard and use your OBDII scan tool to retrieve information.
Since there has not been a regulatory component mandating standardization the various connection types have become proprietary as manufacturers try to maintain their closed end to end systems.
Other Proprietary Systems
There are some motorcycle manufacturers such as Suzuki that have maintained their own proprietary systems. Suzuki's implementation of an on-board diagnostic system is called Suzuki Diagnostic System or SDS. It uses it's own proprietary protocol on the wire. Much like Microsoft did in the 80's and 90's with LANMan and Apple did with AppleTalk network protocols before full adoption of TCP/IP as global network protocol standard.
There are other examples of proprietary BUS network protocols but too many to list. The above list with CAN BUS implementations is not a full list, just citing a few examples of conformity to a standard.
In the future, companies like Bosch, Siemens and NGK are driving standards but it may be quite a few years before full adoption is achieved. Cost reduction for overall compliance is one of the drivers for the hold out manufacturers and with more than likely drive compliance as CAN BUS costs are reduced to the point where lack of adoption does not make financial sense anymore.
Interestingly enough, Google searches for (insert manufacturer name", "can bus", programmer yield a plethora of hits for hiring programmers for Ducati and Harley Davidson Most of the good information is hidden by password protected portals for dealers and certified technicians.