A few more evenings of coding resulted in ability to export custom columns to PDF. There is a new theme Samara which is aware of custom columns. User interface is very simple: columns that you see in the task table are columns that get exported to PDF, plus resources and task notes. Samara theme tries to keep proportions of column widths, places notes under task names, using smaller font and indent and uses task colors in a narrow column on the left, rather than in the whole task row.
A screenshot below shows our HouseBuildingSample project exported to PDF with Samara theme.
Samara is a big and old Russian city on Volga river. I have never been there (except for one sporadic transit through its railway station 13 years ago) but GanttProject feels comfortable there (as well as in many other cities worldwide :)
Update: These changes will be available in GanttProject 2.0.5