Assume a (self-answered) question gets locked for historical significance. Because of this, the user who posted the question cannot (repeat: not) perform any of these actions:
- mark own question as accepted.
- flag the question for moderator intervention.
- add comments to own question.
- delete own question (if that's what OPer would want to do).
OPer can only share it.
My suggestion: Provide an option to be able to mark OPer's own answer as accepted, by OPer (not via whatever moderator / SE employee intervention). So that the system does not block OPer from accepting OPer's own answer (after at least 2 days, as usual).
Note: This is a problem/issue that applies to any SE site, so it is not specific to a specific site. Try a similar scenario on any site, and you'll have the very same issue that the OPer cannot accept his/her own answer. Therefor I think this question belongs here, on Meta.SE.