I have no idea what your tech background is, so please forgive me if I'm explaining stuff you don't need explained.
I use a Unix/Linux *J client called Clive, where I just replaced references to livejournal.com in the source code with scribbld.net and recompiled it. *poof*, instant Scribbld client. Clive works from the command line in a text window ("MS-DOS Command Prompt" in Windows, "Terminal app" if you're a Mac user; actually I use telnet or ssh to log into a Unix machine at my ISP) without even any menus or move-the-cursor-around bits (it does invoke the text editor of your choice, if needed). This makes it ideal for use within a shell script (think "batch file" if you're a Windows user).
I use the Unix 'cron' command to schedule a script I wrote, to run every morning. That script plucks the first quote from my file-of-quotes -- my QotD queue -- and posts it to each mirror of my blog then archives it.
I know there are implementations of 'cron' for Windows (it should be built in on MacOS, and Linux wouldn't seem quite right without it), but I haven't checked to see whether Clive will compile under Windows. There may be another suitable client though -- it doesn't have to be quite as bare-bones as I prefer to work with; it just needs to be able to take comand-line arguments to post an entry without direct user input.
I insert by hand the HTML that the <lj user=""> pseudo-tag expands to when I refer to an LJ/GJ/IJ/CJ/Blurty/Scribbld user, so it looks right but points to the right place each place the entry is posted.
My script is pretty basic; I want to jazz it up a bit so that it can automatically pick a random uncheduled quote to post if there's no quote specifically assigned to the current date. Ifwhen I get around to "finishing" it, I may post it someplace in case anybody else wants to use it.
When I started using the cron script, it took a while for a couple of my bandmates to catch on ... they were a little disconcerted when they realized their way of checking to be sure I'm okay every morning -- see whether I'd posted the QotD -- wouldn't let them know something had happened to me until days or weeks later when my queue finally ran out.