nabla: Yes, that's probably the best rule.

As far as I'm concerned the interface issue is secondary. It should be very easy to create the necessary additional piece images (e.g. take the current images and add a little red dot or something). And even without the extra images, all the information would still be there in the game history; players would just have to be careful to pay attention in certain situations.

But the most important thing is to get the rules right. Now that we know what the correct rule is, the BK implementation should be corrected as soon as possible.