How to select outside a boundary curve?

While cleaning up a 3D scan of a tree, I would like to delete all faces outside of a boundary curve around the roots visible at the bottom. I tried _DeleteFaces, then _SelBoundary followed by _Invert. However, the faces just don’t invert. Also I tried the various selection modes for _SelBoundary, to no avail.

In cases like this I would use a lasso select like this:

LOL, my slow brain didn’t even think about that. Yes, in this case it will do the job. Thanks!

(still it would be nice if there was another option, for when I want to delete all around)