Machining a replacement cog will be rather involved and costly I also suspect. I know of no one doing such currently with rear cogs. There are a very few outfits that make custom chainrings... but all of these I've seen didn't have any shifting aids, just symmetrical teeth. Does your large rear cog have shift gate currently?

The skipping on that large cog could well be from mere wear, the typical chain wanting to ride up the tooth's face more and more as all wears, until the chain finally reaches the tops of the teeth and thus skip over. But skipping can have other reasons that I would want to eliminate before spending big bucks. twisted teeth, poor low limit screw adjustment, improper "B" gap (worst on that largest cog...) among the common other reasons. The freehub body's ratchet could be skipping, have you visually confirmed that the chain really is skipping over the teeth or just assume that? Has anyone else ridden your bike and offered comment on the problem?

I have to admit that your set up is so far from my happy zone that were I to be dealing with this my solution would be quite different. I have zero need or want for such high a gearing as a 9t will get me. I can go 50+mph down the biggest hills in our area by tucking and coasting. A 9x44T ring is about a 132" gear, no way I can spin that at 90 cadence on the flats. So I would never have a cassette with that high a gear. Andy