Not having to hard code the start - stop times for a target is a useful option. Using a minimum altitude above the horizon to start and / or stop the imaging sequence would be a good way to do this. However, I believe the situation is a bit more complicated in that imaging equipment considerations could add to the start / stop time restrictions.
Let's say I don't want to ever do a meridian flip during imaging. Then, in addition to target altitude, I want an additional limit of "don't start until meridian -3 hours (east)" and "stop at meridian +1 hour (west)."
With a sequence setup with three properly selected targets, I can get a full night of automated imaging with no flips. This sequence can be run on multiple nights without making any changes -- even if those nights span multiple months.
This approach means the list of targets is scanned at sequence start to pick the "best target" to image. This criteria is simple -- the target selected is the one closest to its ending time. When that target is ended, the remaining targets are scanned with the same criteria and the second target is selected. When no target has anything to do, the sequence is ended.
It seems to me that both the altitude restriction and meridian restriction are actually global parameters and not specific to a selected target. So the altitude limits and meridian limits would be setup in the equipment profile. Then when a target is created, there would be the normal start / stop times available or the imager could select "Use Global Limits" and the automated target start stop times would be calculated at sequence start.