Thanks James for your reply. Glad to discover that I'm not alone when dealing with these storage appliances and their lack of compatibility with sssd's automatic UID/GID mapping.
I believe my plan is to implement a similar Rube-Goldberg that generates a passwd/group file and serves them up over NIS/ypbind on a separate machine on our network to provide uid/gid mapping for the NAS (since at least our storage appliance still supports NIS)