If one subscribes to the TimelineItemElementCreating and provides custom item elements for the timeline these elements are not used and default item elements are used instead.

WORKAROUND:

Create a custom element provider:

public class CustomGanttViewTimelineElementProvider : GanttViewTimelineElementProvider
{
    public CustomGanttViewTimelineElementProvider(GanttViewGraphicalViewElement owner)
        : base(owner)
        { }

    public override IVirtualizedElement<GanttViewTimelineDataItem> CreateElement(GanttViewTimelineDataItem data, object context)
        {
            GanttViewTimelineItemElement element = this.OnItemElementCreating(data);

            if (element != null)
            {
                return element;
            }

            return new GanttViewTimelineItemElement(data, this.Owner);
        }
}

and assign it to the timeline container element provider:

this.radGanttView1.GanttViewElement.GraphicalViewElement.TimelineContainer.ElementProvider = new CustomGanttViewTimelineElementProvider(this.radGanttView1.GanttViewElement.GraphicalViewElement);