As of yesterday (rev. #10886) it's possible to snap to paths tangentially and perpendicularly, well, at least in some tools. Basically, this is only possible when individual nodes or handles are manipulated like in the node tool, pen tool and pencil tool. I just realized that I should also implement this for gradient editing. Guides should also snap tangetially and perpendicularly already. Note however that in the selector tool this doesn't work (yet), because there we should implement snapping of curves to curves (whereas so far Inkscape only snaps points to curves).
Anyway, have fun with it and please report if you notice wrong or weird behavior. As soon as you snap to a path, then Inkscape will search for normals and tangents. There's no way yet to toggle this behavior, that's still on my to-do list. (I'm thinking of adding pop-up buttons as a child of the snap-to-paths buttons, similar to the pop-up buttons Johan as added for inserting new nodes)