Even if Asana is all about openess and transparency, sometimes you just want to lock down a specific custom field and forbid any change. This is not possible in Asana (yet?!).
Here is the trick to use as a workaround:
Create a project private to you, named « Illegal custom field value change »
In the project you want to watch, setup a rule → when custom field value changes, add task to project created in (1)
If you setup the notifications properly, you’ll be notified of any new task coming into the watch project, and you’ll be able to rollback the value of the custom field by looking at the history trail of the task.
Do you find this useful?!