Does anyone know the syntax to combine multiple custom form fields into a single column on a project report?

Yep. All here 👇🏻

https://one.workfront.com/s/document-item?bundleId=the-new-workfront-experience&topicId=Content%2FReports_and_Dashboards%2FReports%2FCustom_View_Filter_Grouping_Samples%2Fview-merge-columns.htm&_LANG=en

I use this one for last note: note text, date, and owner of note.

displayname=Latest Update

namekey=latest update

textmode=true

valueexpression=IF(LEN({lastNote}.{noteText})>140, CONCAT(SUBSTR({lastNote}.{noteText},0,139),"...(open for more) -- ", {lastNote}.{owner}.{name}," on ", {lastNote}.{entryDate}), IF(LEN({lastNote}.{noteText})>0, CONCAT({lastNote}.{noteText}," -- ", {lastNote}.{owner}.{name}," on ", {lastNote}.{entryDate})))

valueformat=HTML