It would be nice to have a hook on the end of a rall. line, for example. Is it possible to customise the line? We only have three fixed choices.
No, I’m afraid at the moment this isn’t accommodated by tempo items. You can of course hide a real rallentando for playback purposes via Properties and substitute a system-attached line with text at the start and a hook at the end, though it is a couple of extra steps.