I haven't actually used busy.org yet, but yes, that's exactly right. There's nothing preventing any front-end app (that you give your posting key to) from awarding a lower vote weight to a post or comment.

I seem to remember reading that restricting the slider was a decision in Steemit to prevent users with low SP, from using too much blockchain space and bandwidth with what they call 'dust' (very low votes that don't have any impact on payouts).

The API function a programmer would use asks for the vote weight between -100.0 and 100.0% - negatives for flags.