Interesting. I've only seen examples that some museum deemed worth displaying; but at least those ones gave a very, very, convincing impression of being an actual metallic joint; not a gilded lacquer seam. I always assumed that this was in fact the case, and some sort of gold solder with adequate wetting of ceramics had been used.
In retrospect, it seems less plausible. Gold produces pretty dramatic colors when it diffuses into glass, so all but the darkest glazes would be quite a trick to leave unscathed in the presence of a gold alloy solder; and apparently getting solder to wet ceramics without either doing some vapor deposition ahead of time or employing somewhat exotic alloys is deeply nontrivial.
The illusion is very skillful.