Just found this Juniper KB:
https://kb.juniper.net/InfoCenter/index?page=content&id=KB28204&actp=RSS
Quote, "As of Junos 12.1X45, logical tunnel (lt-0/0/0) interfaces are supported on Branch SRX platforms in Standalone mode, and are supported in high-end SRX devices in both Standalone and HA mode.
The interfaces are not supported in SRX Branch devices in Cluster mode."
Looks like they are STILL not supported on 15.1X49-D50.3.
Ugh. 