While reading this, the recent discussion on wikitech-l about bitrot came to mind (https://lists.wikimedia.org/pipermail/wikitech-l/2016-August/086200.html).
It seems to me that one might consider saving the revision id of templates at time of storing a revision on the parent page. This under the premise that people will generally edit again in quick succession if a template produces strange results. So the assumption that upon edit, the page will look reasonable enough to persist that status doesn't seem bizarre (or at least, the representation will be faithful). In principle, if one stores the template invocations (are sub-slots an option here?), the exact revisions and how they look can be recursively identified based on timestamps and the revision history for the templates.
If this would somehow be feasible it sounds like it would alleviate breakage based on template changes, and especially with intricate templates, this can transform the look of an entire page. 't Would certainly be an interesting opportunity to look into IMHO.