I tried a bunch of pens that promised an opaque fine white line on dark paper, and the only one that had an acceptable result was the Uniball Signo Broad. It was in a class of its own, superior even to markers (too chalky) and gloopy paint pens (hardly even work.)
I tried equivalent models from Sharpie (the water-based marker is too thick, and the metal-tube pen just doesn't flow well), Pentel (not remotely opaque), and Sakura (fine in a pinch.)
It wasn't perfect, though, and you'll have to write with more care than normal gel pens. In particular, the pigment dries fast on the ballpoint -- even as you write -- which can result in smudgy or lost corners or thin parallel tracks instead of the expected bold line.
I tried using it as white-out, too. It did OK over Pigma ink (not pictured), but was pretty rough over Higgins ink (below). Reinking over it with Pigma and Tombow pens was fine, but Higgins required a extremely light touch with a Hunt #102 nib.
Uniball Signo Broad [Amazon link]
