controlling [drag-and-] drop in a tree

[Dave Howorth <dhoworth mrc-lmb cam ac uk>]

I'm getting on pretty well with my app. It's purpose is to allow editing
of a tree - in place reordering and insertion and deletion. Insert and
delete work fine. The default drag-and-drop provided by TreeView &
TreeStore is working fine and I'd now like to get a bit more control
over the destinations. I want to prevent the user dragging a node and
dropping it in an inappropriate place (or force the drop onto the
nearest appropriate destination).

I've seen mention of 'gtk_tree_drag_dest_row_drop_possible' and the Perl
equivalent 'boolean = $drag_dest->row_drop_possible ($dest_path,
$selection_data)', which looks like it might be related to what I want
to do. But I can't find any description that tells me where it's
implemented, how it's called or how to override it.

Is there some way to set a callback that is called before a drop takes
place and that can examine and perhaps modify the destination? Or some
other way to achieve something similar?

Thanks and regards,
Dave