Ok--I've got another work around: use VSTHost in "Slave" mode, and the associated VST plugin "Legree" in Abox. Instead of outputting MIDI to a virtual cable in ABox (which fails), stream it to the Legree plugin; process the streamed MIDI data in VSTHost/slave, generating wave data; this wave data is then available in ABox as wave output from Legree, and can be further processed in, or output from, ABox.
What I hoped to do--but have been unsuccessful, however--is to output the streamed MIDI data from VSTHost/slave directly to a virtual cable, which should be possible if I understand the manual.