In essence, can a non-Sysadmin, non-DBO, or non-db_owner role member create objects in the system databases?
Yes. Ddl_admin is more than sufficient.
Do note that TempDB is recreated completely on every start and hence all user permissions and tables will disappear. You need a way to put the permissions (and any necessary tables) back after a restart. Not trivial.
Is this 'operation' something written by in-house developers or is it a 3rd part vendor?
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)SQL In The Wild
: Discussions on DB performance with occasional diversions into recoverability
We walk in the dark places no others will enter
We stand on the bridge and no one may pass