EDIT: UPS. Misread date of post…
I have recently been working on something similar to this… I’m now thinking entire new sensor, and not just additions to existing.
In my experience, modem manufactures do not conform very well to the standard mibs, and I have often seen modems that do not return values.
Planning on using CISCO-DOCS-EXT-MIB (ciscoDocsExtMIB) which contains number of CPEs online/offline and others. AND docsIfSigQSignalNoise which returns SNR value for the US channel (needs to be divided by 10). I would poll the CMTS, and graph these data, since I have no interest in monitoring thousands of modems.