What permissions do I need to grant on service accounts on SQL root path when relocating?
I was able to get guidance from the following MSDN link: Configure Windows Service Accounts and Permissions. The pertinent section (bookmarked in the link) is "File System Permissions Granted to SQL Server Per-service SIDs or Local Windows Groups". 

The short version for the solution is that either Read, Execute or Full Control are needed on the leaf-level subdirs under the system root (DATA, LOG, FTDATA, etc). 

On my host, NTFS permissions weren't propagating from the mountpoint volume root or the folder by which the SYSDB LUN were mounted. Applying NTFS permissions on the leaf-level corrected the issue.