Nic...
I don't work boro, so not sure I can answer your question
If it were me, though, even working in soft glass and wanting to do an off-mandrel sculpture, I start just as you describe with making a gather at the end of a rod (make it a good sized one, too!) and shape conical as you see in Step 2 of Sarah's great tut. Then start adding and building from there.
Keep us posted...
De