Just found this thread. Pretty good idea. I think all 3 versions would make good stickies for this forum.

Anyway, to answer the question, when I was CX Silver (OW Ruby), I was allowed to bring my son into the BA Terraces Lounge at JFK. CX contracts with BA for use of that lounge, so it's treated as entering a CX lounge rather than a OW lounge. But I was able to bring my son in, who at the time did not have status with CX. In HKG, however, they wouldn't allow a guest in as Silver (CX gives its own Ruby members lounge access when flying CX) without the necessary 7500 mile deduction.