you cant lock events to time position...what kind of movie scoring you are doing? nohing frame synced? or do you just live with the inconvenience that you cant alter the speed of the soundtrack without loosing the time position of all frame synced sound events?
When I use Live to score against film/video, I create one project for each individual piece that needs to have a "tempo". For the main project I usually choose a tempo of 240bpm so that each bar corresponds to one second of film. I drop the audio that is rendered from each of the individual projects into the overall project timeline.
The way I see it, it's not very convenient to use a single project for everything. Managing tempo maps is just one part of it. It's just as much a problem to force yourself into having tracks and returns for each piece of music in the whole project -- it gives you a huge track count and makes mixing that much more complicated. Much better I think is to cut off a little piece of video, score to it, render the audio, then go to the next section. It's better organized and it means when you assemble the whole thing you're working with rendered audio (single wave files) for the soundtrack rather than a whole bunch of instrument and audio tracks.
I work the same way in Logic. It's just as much of a problem to mess around with the tempo track in Logic as in Live. Even though you can lock the position of things in an absolute sense (lock them to a frame position), when you change an earlier part of the tempo map, the later parts of the tempo map are all shifted in time, so that if you need to go back to a section and touch it up, you will find that the tempo map is out of place. It's a mess -- better to break the problem down a bit IMO.
Of course, everyone has a different approach...