You can use dynamic enums in Guvnor (that create dynamic populated
listboxes).
See guvnor's doc chapter "1.4.2.4.8. Data enumerations (drop down list
configurations)"
For your case where number of items are really huge, you may consider
using custom editors (again, in Guvnor), and implement a js page that
contains some listbox (as many as level of classification in your
hierarchy) or directly a tree to select the value. See "custom editor"
and "working set" part of Guvnor's doc.
If you are under Eclipse, well, Drools suggestion engine modification
may be the only way.