I don't think you can do this with the standard tools. You can use ipcs -mp to get the process ID of the last process to attach/detach but I'm not aware of how to get all attached processes with ipcs.
With a two-process-attached segment, assuming they both stayed attached, you can possibly figure out from the creator PID cpid and last-attached PID lpid which are the two processes but that won't scale to more than two processes so its usefulness is limited.
The cat /proc/sysvipc/shm method seems similarly limited but I believe there's a way to do it with other parts of the /proc filesystem, as shown below:
When I do a grep on the procfs maps for all processes, I get entries containing lines for the cpid and lpid processes.
For example, I get the following shared memory segment from ipcs -m:
------ Shared Memory Segments --------
key shmid owner perms bytes nattch status
0x00000000 123456 pax 600 1024 2 dest
and, from ipcs -mp, the cpid is 3956 and the lpid is 9999 for that given shared memory segment (123456).
Then, with the command grep 123456 /proc/*/maps, I see:
/proc/3956/maps: blah blah blah 123456 /SYSV000000 (deleted)
/proc/9999/maps: blah blah blah 123456 /SYSV000000 (deleted)
So there is a way to get the processes that attached to it. I'm pretty certain that the dest status and (deleted) indicator are because the creator has marked the segment for destruction once the final detach occurs, not that it's already been destroyed.
So, by scanning of the /proc/*/maps "files", you should be able to discover which PIDs are currently attached to a given segment.