Thanks. I didn't exactly add a new Java field, but I did generate an additional Lucene field based on the id property, simply by doing this:

@Id
@Field(name = "id_forSort", index = Index.YES, analyze = Analyze.NO)
@SortableField(forField = "id_forSort")
public Long getId() {
...

Then I use the id_forSort field for my SortField instead of the id field.