I do a lot of inking on unprimed metal - it's how I do all my metals, where possible (i.e. the figure has to be metal!) and is my main stock-in-trade for minis painting.
Usually using GW inks directly over polished metal, but I'm sure other inks would suffice if they're close enough. I do it in two stages. First I burnish the metal using a wirehead brush with my dremel. You have to be very gentle and not apply much pressure, because it's way too easy to just sand off all the detail on a figure. There's no need to really force the brush into deep cracks, because you'll get ink pooling in those anyway - it's the convex surfaces that you really want with that metallic gleam.
Typically I use Devlan Mud for average metals, Gryphon Sepia for golds, and sometimes Azuremen Blue for a "cold steel: type of effect. I've also managed to get a really cool damasked effect with layered Devlan Mud and Azuremen Blue, but it's difficult for me to pull off. I will also layer multiple washes to get other effects (like many layers of Devlan Mud and Gryphonne Sepia for a coppery effect). In some cases, I do lightly drybrush with a metallic paint to put some coloured gleam on edges. This is usually the case when I need a nice bright/shining or a deep rich gold (I would use Burnished Gold and Shining Gold, respectively).
The main drawback is that the wash scratches fairly easily, so you have to be a dab hand with varnish (I'm not), or just learn to be really careful, which is obviously not for everybody. But if you're doing this for set-piece photos, it's probably just fine.
Here are some examples:
Regular metal

Copper (helmet), Brass (shoulder), Regular (chainmail), Cold Steel (sword blade), rust (light dabs of watery red ochre)

Various golds (armour bits), Damask (sword)

Closeup of regular metal on a stripped & repainted mini, which made the metal slightly pitted (closeup of breastplate)

And an older mini (sorry for the crummy photo) where I gave him a green wash for a lacquered or enamelled look, scratching the ink off the raised parts of the plate armour pattern. Also his swords were a hamhanded attempt at some sort of "blueing".
