Resolution:
Setting the RTprocess option trace_file foo.txt  will cause a file named "foo.txt" to be created for the RTprocess' trace output.
If the option  setopt backup_name .bak  is set after setting the trace_file name it appears to be ignored. So, if the RTprocess is started, stopped and restarted, the name of the trace file backup will be "foo.txt~". ('~' is the default backup_name)

To force the correct behavior, the backup_name option *must*  be set before the trace_file option  or any other trace file option for that matter.
/* Example of correct usage */
setopt backup_name .bak
setopt trace_file_size 5000000
setopt trace_file foo.txt
setopt trace_level debug
setopt trace_flags prefix,timestamp
setopt time_format full