Tip: When determining a time limit, remember that players won't be as familiar with the level as you are. If you normally complete the level with around 100s left on the timer, others might run out of time on their first try.
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.