I assumed too quickly that the issue was resolved. I should have checked to see if the ReadyNAS files were visible through Tivo (they were not). I also should have thought to have support reboot the NAS to confirm that the issue would not return then.
When troubleshooting the Tivo listing on my own, in Frontview > Streaming Services I rescanned the media files, disabled and enabled the Tivo service (with no result) then rebooted both the Tivo and the NAS. That took it back to the original problem condition -- I can't see the files in the "media" directory via CIFS. Again I await escalated support to access the system.
I assume that this is not related to v4.2.19, but instead some config file which has an effect at bootup getting inappropriately modified by the installation script of either subsonic or Plex Media Server (which the logs show me I installed the same day as subsonic). But I am just guessing.
ReadyNAS NV+ v2, RAIDiator 5.3.6, 4x3T Seagate ST3000DM001-9YN166 fw CC4H, X-RAID2, 256MB RAM, UPS APC BE550G ||| ReadyNAS Ultra 2+, RAIDiator 4.2.20-T4, 2x3T Hitachi HDS5C3030ALA630, X-RAID2, 1024MB RAM, UPS CyberPower CP550SLG