A journal of the type you've described is technically a series of short writing assignments, AKA "writing to learn."
Wherever possible, I try to design learning activities by using the basic "out of the box" functions of Moodle, i.e. without plugins. One way I can think of to do a student journal is for students to write them on a single Writer/Word doc that they submit via the Assignment activity module. They then have their own off-line copy that they can review & edit at anytime and then resubmit with new/subsequent entries appended. You could even provide a pre-formatted Writer/Word doc with all the specified journal submission outlines & dates for the whole course.
This method would make giving participation grades, using rubrics (writing to learn implies a heavy workload on teachers giving feedback so well-designed rubrics can help to reduce that), and/or quality grades easier.
One drawback would be that students & teachers may mix up older & newer versions of the journals because this strategy would generate a lot of copies from uploading & downloading.
In the spirit of sharing,