Equivalent of AutoCAD "boundary" command? Or Boolean ops?

Hello,

I would like to be able to create arbitrary shapes by, in effect, "filling" any visible boundaries/shapes, as with the AutoCAD command "boundary".  
This is *like* but definitely not the same as hatching, because the visible area to be filled need not have endpoints at intersections etc.  
It's really just like the Paint Bucket tool in GIMP/Photoshop/paint etc., where a polyline is created in the shape of whatever area you click in.

I'm not seeing reference to such a function anywhere in the docs or inter webs.
An alternative is to use 2-D boolean subtractions/unions, which I also haven't found (yet?).

Any help would be greatly appreciated, as either of these two methods would be a massive step to allowing LibreCAD to obviate all my other CAD software!

Thanks!