@Matt_Bramlage Hey Matt, these guys are correct - I just tried the CURL syntax you suggested above (and a bunch of variations of it) and got the same error.
For example, given a task with a text custom field having id 744020393575926, if I do:
curl --request PUT -H "authorization: Bearer MY-PERSONAL-ACCESS-TOKEN" https://app.asana.com/api/1.0/tasks/718125926216046 --data-urlencode "custom_fields.744020393575926=A field updated from CURL"
that results in the error Could not interpret 744020393575926 as an identifier in custom_fields.744020393575926.