In my experience free writing doesn't work as well, because some kids have a hard time deciding what to write about and spend so long trying to choose that they don't have much time to actually write. Since your daughter's schedule is tailored more for her individually it might work better for you than it does in a larger class. If you decide to assign specific topics,
this link might be helpful to start you off. I was googling for the list my son's teacher used a few years ago and couldn't find it, but there are many similar topics on here.
I haven't really kept up with the Whole Language info online since I graduated, so I don't know any good links. (Actually, I didn't keep up with any info about it online when I was studying it - we had email but not much more than that!). I'll poke around and see if I find anything similar to what we learned - it wasn't exactly like the method most people seem to write about. The thing I loved about it, though, was that it seemed like common sense. It made sense to me that you'd approach the education process as a whole rather than segmenting it so much. There wasn't really much actual "method" to it; it was just a breakdown of the lines between the classes. Kids learned without realizing it, if that makes sense. Since they tend to excel at different things, they seemed to do better when the subjects they didn't feel comfortable with were blended into the ones they did like. So the kids who were strong in science ended up being more comfortable with the English lesson because they were writing and diagramming sentences about science, which they did like. The English kids absorbed more about science because it felt more like an English lesson to them.
I don't know much about Homeschooling but I think that lots of Homeschoolers do this without knowing that it's a form of the Whole Language method. One person I know is doing a unit on Christmas cookies right now. All of her lessons are themed to the cookies. I don't know if this is part of the actual curriculum she follows or if it's something she came up with on her own, but her kids are loving it and apparently it's going really well.
I love the trend to include writing in more of the other subjects because I've seen too many people get to high school or even college without feeling comfortable with writing. These days writing is so important, even in many jobs, that it's a huge advantage when someone really feels comfortable putting their thoughts on paper. And of course journaling is an excellent thing for kids, even aside from the educational aspects. It's a great outlet for things that they are thinking about but don't necessarily know how to talk about. I think it also helps them learn to express themselves well when they can actually look over their words and re-read them, and re-phrase them until they say exactly what they want. (I imagine the dictation method you use is also good for that.)
I can remember times when my son would sit for hours staring at a blank sheet of paper trying to figure out what to write. After a few years of journaling, the rest of his writing now seems so much easier. He had to write a 500-1000 word paper the other day. Even a couple of years ago it would have been a multi-hour struggle to get him to write it, complete with tears and a tantrum. Plus he would have stopped at word 500 and called it finished. I was thrilled that he finished the thing in around an hour and it was longer than it had to be. I totally believe that I have all that journaling to thank for it!
ETA - Yes, typically we also did units that lasted a few weeks. I loved it. I think the kids learned more because they were so engrossed in whatever topic we were convering rather than being yanked from one subject into something completely different.