It would seem that the consensus is "no" to having programmatical interactions with third-party plugins as on-topic. The grounds for this answer seem to be that knowledge of said plugins would be needed. However by putting them out of bounds such questions have nowhere to go but back to SO. That they are readily answered which suggests that the required insight does exist within the wider developer community but causes confusion as to why WordPress questions are still on topic for SO. I have a proposal that might solve this.
Would the community be willing to get behind a proposal for a mixed WordPress SE focused on coding for and with WordPress and associated frameworks (such as third-party plugins); a sort of WordPress hacker's QnA (in the most positive sense)?
Such a site would draw away most recurring discussions of third-party plugins and provide a migration path for many off-topic development questions. WordPress hacker's QnA could deal with doing weird things in edge case scenarios with third-party content while this SE could remain pure WordPress development which is what the general community desire seems to favour. Moreover, there would be less need to have SO carry the load with quite so many WordPress specific questions.
Less confusion; more Q and A.
With support from this (established) SE community, WordPress hacker's QnA could take a whole bunch of (what I imagine are) annoying issues away. The degree to which not-on-topic areas are on topic in the new SE could become someone else's problem.
TL;DR: Would you get behind a SE that covers things that are partly or wholly off-topic here?