I gave up on jsonapi with swagger/openapi.
OpenAPI 3 (not yet released) claims to support a few more of the json schema elements, including oneOf. I'm unsure whether that's enough to enable jsonapi.
Also, bear in mind that tools to actually use OpenAPI 3 will naturally lag that release.
I would love to see full convergence of openapi, json schema, and jsonapi, with a good example!