+1 for the idea.
Thanks for the temp fix @julienmelissas
In my opinion, could you make the expand/collapse state just save its state each time you hit the update button. Much like a metabox does when you expanded/collapse it.
If you collapse a repeaters rows and hit update, when it reloads or you revisit the post it would be collapsed.
Then obviously if you leave it expanded and you hit update, when it reloads or you revisit the post it would be expanded.
It would be nice to be able to force a certain state as well if you wish – “Always collapsed / Always expanded” as @adamwalter mentioned