It is possible - you are, after all, writing a separate program that could do just about anything.
* Some client-server models don't allow such interaction.
* You are adding arbitrary data to the socket CVS is using for its data, and could accidentally trigger some other CVS command.
* You could generate a deadlock, as one person's commit can't proceed because it's waiting on another person doing their editing, but the other person is waiting on the first to do editing. This might or might not be a problem in a loginfo script, which runs after the commit is processed, but would be a problem in a verifymsg script.
In addition, CVS doesn't re-read the log message after a loginfo script, so any changes you make are ignored. It can re-read the log message after a verifymsg script, however. (See my previous response.)