LJ (which is what I use) has a downloadable client, which presumably would upload the posts from a saved file that you create. I haven't used the client, myself.
If you're getting logged out after a certain amount of time, it's not the site, as I stay logged in all the time. IME workplace defenses generally revolve around firewalls and/or proxy servers, which aren't going to affect the amount of time which you can stay logged in. More likely it's your browser settings, either cookies or privacy or some other setting that is preventing the login information from being saved. I would check those settings, and also check any support forums or FAQs on LJ to see if the issue is addressed.
I pretty much use LJ by default, myself, since it was what all my friends were on. I haven't looked into any other sites.