Well, it is not just a question of Kaya wanting to come to Europe, it is also a question of organisers thinking that he has enough fans to recoup the cost of bringing him here. As J-music has become more recognised in Europe, it has been taken on by bigger, more professional organisers, who aren't as much fans as they are businessmen and who are therefore playing it safe. If you look at the bands that have been brought to Europe post the "DIY" era of EOD and BLOOD, most are pretty mainstream compared to Kaya (and those that weren't largely flopped, e.g. Celestia Le Ciel and Kana).
As much as I hate cons, I think the best chance for him would be to perform at some large anime and manga con since that pretty much guarantees a good size crowd. It worked in the US, so it could work in Europe as well. Perhaps Japan Expo, since the French seem to be more open to J-music that isn't rock based (for example, I doubt that Kokia or Kanon Wakeshima would have been offered record deals in Germany).