I think this should work. At one point jnxOperatingCPU OID was not supported in chassis cluster mode. But that should be supported by 9.5R1. I would recommend opening a case with JTAC and have them investigate this.
Just to follow up in case anyone else runs into this, I did open a case and it appears to be an issue with this release on the J-series chassis. Apparently the same release for an SRX chassis works fine. So, known bug for now.
Actually I found out that this is supported on SRX. But for J-Series this required an enhancement request. There is already an ER opened for this and is currently under review. So this should be included in a future JUNOS release TBD.