Unfortunately currently I have not enough information about this problem, but I'm sure there is some problem or problems.
1) multiple times I observed very long remount-ro (mount -u -o ro) for some partition with a lot of disk activity from mount program during that. All that was some time ago and currently I forgot details. I'm not sure that this issue still exists in 11.2-RELEASE but i think it is related to next one.
2) Recent one:
"mount -u -o ro" was hanged for a few second and then wrote "mount: /path/to/mountpoint: Resource temporarily unavailable"
Second attempt of "mount -u -o ro" resulted in deadlock in "suspwt" state of mount program. A lot of processes was deadlocked in state "suspfs". Not sure which of them appeared first.
There was no disk activity (in gstat) for remounted partition during this deadlock.