Something silvery.

Sambucus Aurea (Golden Elderberry)

I've heard that other elderberrys can be invasive with their suckers... is golden elderberry like this?

I've never seen a shrubby (woody) elder of any species spread below the ground. I may have seen a bit of layering a few times, although I can't say when or where.

Madonna variegated golden elderberry would be a good one as well. Its the same species (sambucus nigra) so should help with setting fruit. Black lace might be a little less vigorous than other elders because of the cut leaves. I'd like to try one out, but i don't want to mail order just one plant.

No ideas to offer, but wanted to say just make sure you don't plant too close to the box. Maybe check city ordinances about this too.

On my street a neighbour planted a garden to block his view of an electrical box, but when the city came to do maintenance on the box a few years later, they cut down a beautiful globe blue spruce and some spireas (and trampled his perennial bed doing it), because they were within 3' of the box. Apparently it was a city ordinance that none of us knew about.

I planted a pink beautybush (Kolkwitzia amabilis) next to mine. I thought the pink flowers of the beautybush would look nice to the dark foliage of the Black Lace.

Here is a link that might be useful: Beautybush

That was nice that they cut down that expensive spruce without even giving your friend a chance to move it first. Can't they send out notices?

I've read before that elderberries often have a somewhat rustic appearance. How "coarse looking" are this elderberry's branches during the winter?

I can't address the tall companion question, but I found alchemilla mollis growing up through the branches is lovely. And I imagined a rich pink something, like the flowers of the dark elderberry, might be nice...I'm trying a pink columbine in back, but it is too pale.