I agree with you on the time piece. I remember wishing I hadn't assigned such robust Social Studies projects when I was spending my entire weekends and evenings grading video projects with a three page rubric!
It's a tough line to walk. Teachers often slam the MEAP because it doesn't assess students at a deep enough level, however, we realize quickly (like you and I have) that the time it takes to assess students at the apropriate level takes a lot of time.
You've probably tried it, but have you had any luck with assessing writing while focusing on one or two areas? I found a bit of success with it last year when my students knew I was only going to be assessing organization (particuarly leads and conclusions) because my lessons focused on those areas.