You mentioned that your usecase is to keep track of a user-provided bbox even if it is not detected by PGIE. However, all the trackers in DeepStreams are designed to work in conjunction with a detector that is expected to provide the object metadata whenever detected. So, unfortunately your usecase is not supported right now, unless you manipulate the detector’s output (i.e., tracker input) via a probe.
Also, as you can find more information on custom low-level tracker in DS Doc, all the input/output data are defined by an API in sources/includes/nvdstracker.h. So the access to user_meta is only available in the plugin, not the low-level tracker library. This means that even if you implement a custom tracker, the user_meta can’t be provided to the low-level tracker unless the APIs are modified.