Setting player selection endpoint?

[Gweneph]

I'm wondering if there's any way to set the end point for a selection tool.

In the API there's LuaPlayer.start_selection which allows you to modify or start the selection of a selection tool, but it always completes at the current cursor position, which doesn't seem to be modifiable even through LuaControl.update_selected_entity which takes a position. So is there any way to activate a selection tool with both the start and endpoint under mod control?