Next update:
OK, so the issue is definitely related to some sort of interaction between the CIFS mount and the log file being locked. I'm working with support, and the next step was to add a line in my sasv9.cfg file to disable file locking on the directory I was sending my log to, and that did open up the file for viewing. Of course, this is not a permanent solution since disabling file locking is not a good idea, but it did help diagnose the issue.
So I guess the question now becomes "is it possible to disable file locking only in the context of proc printto" OR "what the heck is going on with CIFS that it's essentially locking the file from both read and write and how do I fix that?"
I'm still working with support but just wanted to provide updates as they come.