Sorry for the confusion. I'll try to explain it better. I avoid overriding the default engravers because I utilise many of them at the same time attached to the same note or context. I cannot afford to modify the default arpeggios because I try to implement things with a bit more flexibility. e.g. I may need a standard arpeggio but also need the customised one (simultaneously). For horizontal spanners this isn't a big issue since David Kastrup created a code that adds as many as needed. But for vertical spanners there aren't many options.
To give a bit more context on why I need a vertical spanner that uses the note positions for its extent. One way of notating accidentals for clusters is to use stretched out accidentals that span the whole cluster. Instead of using lots of manual scale tweaks I'm trying to optimise my process by having Lilypond do the hard work. Now I'll need to experiment with mapping the stencil to the spanner extent which I've done for other kinds of spanners.
This spanner will be used for other things as well so it's good to have options.
I wish we could arbitrarily make spanners that inherit the properties of the default ones but can co-exist with them. Now we have to define the whole alternative engraver from scratch.
Best,
Dimitris