What you're basically saying is "for the next six months I won't have much time to write my novel, so instead of writing it or doing planning and research that's directly necessary for it, I'm going to write this other thing instead that I don't have to write" - can you see why that comes across as a fancy way to procrastinate?
A journal doesn't need to have an arc, a plot, interesting dialogue, or consistent voice. I can just experiment and try different things until I find something interesting. I will be working on the story but I won't be actually writing it yet, if that makes sense. It's sort of like doing research except I have to make up the facts first before I discover them.
Experimenting as research is fine, as long as you really are doing it to develop character voice and as part of your research and planning, and it's not all an elaborate way to procrastinate. Do as much of the journal as you need to to develop your character. But don't get into this thinking like "I don't have time to work on my novel so I'm going to do xxxxxx instead." That's not going to help you at all, it's just going to delay you in doing the actual work needed to start the novel, which is the very last thing you want to be doing when you have so little time!!
When I write a novel the story is on my mind all the time until I finish,
Most writers have to do a day job and many have families and so don't have the luxury of being able to devote a lot of time in the day to writing. I'm a single mother who works full time. If I put off any serious projects until I have large amounts of time each day to write it, then I probably won't get anything finished until I'm in my 80s. I totally understand what it's like to have ambitious writing projects and very little time to do them. But the ***only*** way to deal with that situation is to get on with doing the writing projects (includes the writing, editing/rereading, and any research that's necessary to write it) I don't do outlines, but if I did, then I'd schedule in time in between work, my kids and all the housework to do that too.
and I want a plan before I start writing. I can write a journal entry in a couple of minutes and if I don't like it I can just disregard it.
You can do that with scenes of a novel. Write a scene, half a scene, a paragraph, a sentence... if you don't like it, disregard it and write it again. You don't have to write an entire novel all in one go before you evaluate anything. You need to evaluate it bit by bit as you go along.
And if there are larger sections of the novel that you don't like, they too can be rewritten. No-one writes a perfect novel first time, barring the occasional very rare genius. You write it and the first version usually is pretty rough and needs a lot of work to beat it into shape. You say elsewhere that you don't want to write it when you don't have much time because you don't have experience editing and revising. Well, how are you going to get experience if you don't do it? You write the novel. It comes out full of drivel. You edit out the drivel and rewrite what needs to be rewritten. And then you have experience revising and editing AND a novel that isn't full of drivel any more. Win-win.
If you need to do an outline first to be sure that the whole story's going to be coherent before you begin - do the outline. Start it now. Don't wait. It can still be done even if you only have a short amount of time each day to do it.
Also, are you really going to only get a few minutes a day? Can you make time? Maybe get up an hour earlier and write for an hour while you eat your breakfast before you go to this course? Maybe schedule another hour before bedtime. Or go to bed earlier so you can schedule two hours in the morning (or later for two hours at night if you work better at night). Your course doesn't schedule lectures/workshops/whatever you're doing at 11pm at night, does it...? Two hours a day is all that most writers get, at least those of us (the majority!) who have to do day jobs and/or have family commitments.
You can also get lots of thinking/mental planning time during the day. I'm mentally planning my next scene while I'm stuck in traffic on the way to and from work - also between calls at work when it's not too busy (I'm a call centre phone minion) and in my lunch hour.
Again, I'm not pooh-poohing the idea of doing experimental writing to help you find your character's voice. What I'm concerned about is that you're putting off starting the project on the grounds that you don't have much time - i.e. procrastination. Less time = you need to get on with it and not get sidetracked by unnecessary tasks.