Some thoughts off the top of my head:

It might be just as easy to create a small command line program that knows how to talk to the blog API, and since there are open source clients that already do that, there is even sample code to look at. Maybe one of those clients can even be run from the command line, I don't know.

I had already thought of this. Googling for it did not yeld any result. My next step in this direction is to contact blGtK's author and ask him if it can be run from the command line.

I can certainly see the need for what you are doing. In an ideal world, everybody would use their favorite text editor / word processor to write everything,

Exactly. It's not an ideal world; it is what, as I said in the other answer, what we already do with email. Yes, I know it's not exactly the same case, email is not co-authored etc... but what surprised me was to find nobody is trying to go in the same direction with CMSs.

ciao, O.