Can Apple Watch Ultra be 24k Gold Plated?

The first reason this is Not a Good Idea is that the watch would need to be completely dismantled to avoid damage during the plating process (and then reassembled afterward). This would all be done buy people not trained to dismantle Apple Watches.

The second reason is that Titanium has some unusual characteristics, and I suspect getting a good and durable result with the plating is far from certain.

Not by Apple and having anyone else do it, would void your warranty, so I'd say no, no it can't.

There is no guarantee. It is totally up to you and your responsibility if you do.