I do not know if you can write to those files, although I suspect you can't.
A possible workaround, if this is just for debugging, might be to have your mod do something that will show up in those files. For example, you could buff yourself with a scroll of agility, and have the mod cancel the buff at a certain time. Then your log would say, "Agility fades from you" or something to that effect.
I ended up capturing the messages sent to the combat log and saving them myself. I'm just keeping the last 2000 entries (combat log + my debug messages). The problem with this approach is that if Blizzard adds a new combat event, I have to modify my code to make a copy of that message. If I could insert my debug messages into the stream that eventually gets written to the combat log, then I've got everything in one location.
While I don't have a need for it now, I'd like to be able to do the same thing to the chat log without having to "say" my messages (which I believe does work).