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I am not a strong Shogi player, but let me relay a comment from the famous Shogi vlogger Hidetchi, which seems relevant here. Hidetchi once commented on a proposal to let the Gold General promote to non-royal King (I think it was in the now dissolved 81squareuniverse forum). His remark was that that this was pointless, as it should have no significant effect on the game. Shogi is a 'race to mate', where every tempo counts, and dropping a general in the zone, moving it to promote, and then use it to capture something with it (benefitting from the newly acquired moves), takes unaffordably long. The overwhelming majority of dropped generals would never move again for the remainder of the game, and certainly not backward. Typical use is to drop them in front of the enemy King with check, either as a sacrifice to draw it out, or forcing it back, and then drop a new piece with check on a (now protected)  square in front of the dropped general.

To have an impact you would have to add extra forward moves, which could check a King after it evaded the checkdrop. E.g. fR, which sounds very powerful, but since it can happen only in the zone and the other moves of a Gold cannot easily pull it back, in practice only will have very limited forward range, of which the first step was already be reachable by a plain Gold. So it gives you one or two extra (but blockable) moves. If you don't want the promoted piece to be upward compatible, FfR could be a good choice.

On my display the static image of the setup wraps (together with the text) around the Interactive Diagram, and as a consequence is displayed south-east-east of the latter. This looks a bit ugly. I would recommend putting the static image in a separate <div> section, so that it will always be displayed on the left margin. I am not sure why you would want to show two images that initially look identical anyway. When I convert a page to using an Interactive Diagram I usually put the static image between <noscript> tags, so that it would only appear when JavaScript is switched off, and the Interactive Diagram would not. And the mnemonic pieces are shown in the Pieces section of the article anyway.

You set up the page such that clients with JavaScript switched off can never see the appendix, as for those the button would not work. It would be better to load the page with a normally visible appendix, but embed a small JavaScript program to hide that. And the reverse for the Interactive Diagram, to prevent the definition would show up for those with JavaScript switched off. The Diagram script contains a convenience routine to toggle the display style of a HTML element with given id, which you can call for this (and also for the button): <script>OpenDiv('appendix');</script> would do it.