The short answer is: use 'epilogue' (var. 'epigraph') regardless of where you place something that is not quite part of the text of the body of the work. It's an 'off-label' use, but then if one is a writer they have license to do just that - especially where no other good devices or terms are provided. Language is inherently flexible in that respect and for good reason.
The longer answer: It happens that "official definitions" of terms often conflict with the actual utility of a word or the lack of substitutes for an 'off-label' application. Professional writers run into this problem all the time and generally ignore the constraints some think are written in stone and not to be violated. But we do it with reason and careful consideration. Bottom line, coinage of usage as well as coinage of terms is an oft used tool on any writer's workbench.
'prologue' (var 'prologomena/-on') unfortunately has been saddled with other formula that require it be some kind of "introductory descriptive" text related to the remainder of a work and, therefore be put at the beginning of a work. The Greek root, 'pro-'/ 'before', supports this constraint.
In the case of the term 'epilogue', the constraint that holds it is only appropriate to use at the end of a work defeats one of its more valuable applications as a general 'meta-textual' indicator that might appear anywhere in a text - something that is merely aside, above or near to the main text of a work, but not quite within it. 'metalogue' might be a good substitute coinage, but it has other uses that are not relevant. 'paralogue' might be another good coinage, but I find it a bit awkward as well. My solution is to simply call these little above-the-text asides (which are mainly used at the beginning of a chapter or section within a work) 'epilogues' as well. The Greek roots of the word ('epi' + 'logos') would support this usage, 'epi-' simply meaning something not within a text, but close to or near to it; 'logos' simply meaning 'saying' or 'thought'.
an example: I have a long work with the following structures:
- A title page
- a number of prefacing pages (index, forward, cast of characters, etc.)
- a one page prologue that contains some descriptive and mood setting material and is a proper part of the body of the work.
- after that follows a page with a brief epilogue, absolutely vital to the work, but not quite a part of it either.
- several other epilogues appear throughout the work (generally, but not always, at the beginning of a new chapter).
- at the end of the work there is also a final chapter that is titled "Epilogue - Auld Lang Syne".
All of which works for me and, more importantly, works for the work itself. Keep in mind as a writer, reality is what you say it is, not what "They" say it must be.
[ps - those who provide sarcasm or 'uhm-duhs' for responses to a question are generally little more than ego-wavers who have nothing to say, but say it anyway. Pay them no mind.]