Thanks w0w… very useful, that gave me what I needed to figure it out.
The issue was that python was not executing.
The so called "portable" shebang that worked at the command line
#!/usr/bin/env python2.7 - DOES NOT WORK from cron.
I created the following file as /home/custom/bin/tcron
#!/usr/bin/env python2.7
import os
os.system('/usr/local/bin/minicron')
When run from the command line, it put the minicron error into the log every time it is run, but did nothing when run from cron.
I changed #!/usr/bin/env python2.7 to #!/usr/local/bin/python2.7, and now it works.
I don't know if this is intentional that #!/usr/bin/env python2.7 doesn't work from cron, but for now I'm not going to worry about it.
I hope by documenting this it might save somebody else the same trouble, and if it's a bug that should be reported, someone who knows how to do that will do so.