How do you send personalized information to each student in Moodle?
Here's a quick thought based on a different learning management system (LMS). Sometimes in other LMSes you can upload feedback to assignments: you batch download each person's file, add comments, and upload each file with exactly the same name. Perhaps you could try a workaround like that, perhaps requiring you to create a fake or trivial assignment and get everyone to submit it.

For a more general approach that is LMS-independent, you could use a spreadsheet and your email program with mail merge. That is, one field would be email address and another field would be the comment, and optionally you could have the person's first name in another field. Then you could send a generic message with blanks filled in by the program. (You could send out a practice message to your own addresses or without any personalized content beyond student names and you can check your Sent log to see if it worked right.)

(The latter solution defeats the purpose of a LMS, but limited feature sets do, as well.)