The command name (at least on my computer, 2006) for dragging in a block from the design center is "DROPGEOM". Here is the code I used to see what it did.
(if (not GlbReactorCommandEnded)
(setq GlbReactorCommandEnded (vlr-command-reactor "InsertReactor" '((:vlr-commandEnded . "InsertCommandReactor"))))
)
(defun InsertCommandReactor (React Lst)
(print React)
(print Lst)
)
Once this is loaded into a draiwng, it will print the name of every command used, until closed and opened again.
Edit:
I only got prompted for insertion point. My company sets up my system variables, and I don't use the design center, so I couldn't tell you why it works like this one my setup.