I think an interesting additional feature for the “prune -keep” command would be the possibility to exclude tagged revisions. This would allow for a tagged revision like e.g. “BeforeUpdate” or “KeepForever” which would never be automatically pruned.
Probably the easiest implementation would be to “prune -keep” only numbered revisions. Or one could implement a regex search string for revisions excluded or included in the -keep list.