I think I understand what you're after. Let me try and restate it so that hopefully we're on the same page:
When adding to a selection in Acorn, if you start your additional selection inside the existing selection, you don't see an outline of what you're new selection is going to be until the selection expands to a point to where new pixels are being selected.
Is that correct? And you'd like to see an outline of the selection tool even though it'd be showing already selected pixels?