I would think you could chain together
- Workfront - Watch Field, specifying 'Project' as the record type and your custom field. Return the project owner ID, project name and the custom field value
- Workfront - Read a record, record type 'User' and return the e-mail address
- Whatever mail system you use, send an e-mail to the e-mail address returned in 2 with the Project name and field value from 1
This would need a flow per custom field that was updated.