Tip: You can use Lunar Magic's Overworld Layer 3 Editing system to edit or create your own layer 3 images (castle bricks, fish, rocks) and apply unique effects per level via the Layer 3 Bypass in the main editor.
Vaguely similar to the "Rupoor" from the Zelda series (Phantom Hourglass comes to mind).
Plenty of settings that could be adjusted to fit a hack's individual needs and probably would work reasonably well in a hack where coins mean more than lives or score. As a personal request, I think it'd be nice to have versions that accommodate 3/6/etc.-digit coin counters as well, but it shouldn't be hard for anybody with basic ASM experience.
Also, the bug that you fixed also fixes a possible glitch where if you have exactly 1 life shown on the HUD, and keep collecting the anti-coin, you end up with 0 lives shown without dying, which is lower than the minimum number of lives the game would allow before a game over. If you keep collecting them, you'll end up with negative lives ("P5") also without dying.